LONDON (AFP) – Airlines urged a re-think of flight restrictions as a volcanic ash cloud made Europe a virtual no-fly zone for a fourth straight day Sunday with nearly seven million passengers affected.

However, the European Union presidency offered a glimmer of hope for stranded passengers, saying half of the flights scheduled for Monday in Europe could likely go ahead.

Several European carriers launched test flights to challenge warnings that the volcanic ash cloud spreading across Europe from Iceland's Eyjafjoell volcano since Wednesday would destroy jet engines. Air France and KLM reported no problems.

A grouping of the continent's 36 major carriers called on governments for an "immediate reassessment" of the restrictions, saying they were having a "devastating impact" on the industry, and questioning whether they were proportionate.

"Airlines must be able to fly where it is safe to fly and make decisions accordingly. It is what our passengers demand of us," the Association of European Airlines said.

Meanwhile, KLM declared European airspace "safe" after a series of test flights, as the Dutch traffic authority allowed KLM to fly two cargo flights to Asia and the Middle East.

Air France said their first test flight, an Airbus A320 flying from Paris to Toulouse, "took place in normal conditions. No anomalies were reported."

However, French Ecology Minister Jean-Louis Borloo said the test flights were not steps towards reopening European airspace.

"We do tests and everything goes back to normal? No. It's not a theory which exists."

Two of Europe's three biggest airports, London Heathrow and Paris-Charles de Gaulle, remained shut.

But German authorities authorised the other, Frankfurt, plus five other national airports, to resume a limited service for several hours on Sunday, depending on the flight's direction.

Airports that had been closed in northern and eastern Spain, and airports in southern France, also reopened.

"Forecasts are that half of the flights planned (in Europe) could take place tomorrow," Spanish Secretary of State for European Affairs Diego Lopez Garrido told a press conference Sunday at the Eurocontrol flight coordinator.

However Britain later extended its flight ban until 1800 GMT on Monday and France ordered most airports shut until Tuesday morning.

About 30 countries have closed or restricted their airspace due to passenger safety fears, snagging 6.8 million passengers in a global backlog, according to the international airports council, ACI.

"More than 6.8 million passengers have been affected so far and European airports have lost close to 136 million euros (183 million dollars)," said its European head Olivier Jankovec, adding that a total of 313 airports had been paralysed by the restrictions.

More than 7,000 transit passengers have been stranded in the Gulf states of Dubai and Abu Dhabi, airlines said. Dubai-based Emirates Airlines said it was "providing accommodation and three meals a day for more than 5,000... transit passengers at a cost of more than one million dollars per day."

In Singapore, Changi airport and airlines have set aside special areas for stranded passengers to eat and sleep and are trying to provide them with free meals, sleeping bags and even tours of the city.

The enormous shroud of fine mineral dust particles now stretches from the Arctic Circle in the north to the French Mediterranean coast in the south, and from Spain into Russia.

The cloud is now heading toward Greece and into Russia, weather experts said.

"This is one of the most serious disruptions we have seen to air travel and our first priority must be the safety of passengers," British Prime Minister Gordon Brown told BBC television.

"The most important issue is making sure that it is safe to fly."

Some governments have set up emergency cells to work out how to get tens of thousands of stranded nationals home.

Around 63,000 flights have been cancelled in Europe since Thursday, the continent's air traffic coordinating agency Eurocontrol said. Some 20,000 flights were cancelled on Sunday alone, according to a midday count.

Iceland's Eyjafjoell volcano started erupted on Wednesday, sending ash drifting towards Europe.

European Union transport ministers are to hold videoconference talks on the volcanic cloud on Monday with some officials expecting demands for special aid for airlines.

EU Commission chief Jose Manuel Barroso said he had ordered a full study to assess the impact of the situation created by the volcanic ash cloud on the economy, and the air travel industry in particular.

The closures stopped world leaders, including US President Barack Obama, from flying to the Polish city of Krakow for the state funeral of president Lech Kaczynski and his wife Maria.

Airlines complained that European governments were overreacting to the threat, relying on incomplete data from computer models rather than real-world safety tests in the air above Europe. In a blunt statement Sunday, representatives of Europe’s airlines and airports called for “an immediate reassessment of the present restrictions.”

An Indian mother and her children rested in the transit area of the Frankfurt airport on Sunday./Pool photo by Boris Roessler

Europe’s transportation ministers decided to meet in Brussels on Monday to discuss how and when to get planes back in the air.

“It is clear that this is not sustainable,” the European Union’s transport commissioner, Siim Kallas, told reporters in Brussels. “We cannot just wait until this ash cloud dissipates.”

Europe remained a scene of travel chaos, with deserted airports and grounded planes; stranded travelers stormed ports and bus and train stations. London’s St. Pancras train station, where Eurostar trains leave for Paris and Brussels, was packed with people anxious to find a way to Continental Europe.

Angela Merkel, the German chancellor, finally arrived back in Germany from San Francisco, after a three-day odyssey through North Dakota, Portugal and Italy via plane, armored car and bus. And one group of intrepid Samaritans tried to evacuate stranded travelers by dinghy from Calais, France, to Dover, England.

Travelers in front of a board full of cancellations at Charles de Gaulle Airport in Paris on Sunday./Michael Kamber for The New York Times

The closing of European airspace has dealt a severe blow to the beleaguered airline industry. The crisis has cost the airlines at least $1 billion so far in lost revenue and could wipe out weaker carriers if it continues much longer, analysts say. Airlines have already suffered losses of $50 billion over the last decade after the attacks of 9/11, the SARS epidemics of 2004, the rise in fuel costs in 2008 and the recession.

Authorities are concerned that if an airplane moves through the ash cloud, which contains high levels of silica, a glasslike dust, the engines could seize or stall.

But several airlines, including Lufthansa of Germany and KLM of the Netherlands, completed successful test flights on Sunday, and said they saw no damage to their planes. The chief executive of British Airways, Willie Walsh, hopped aboard a Boeing 747 flying from London’s Heathrow Airport to Cardiff, Wales, to gather data on the ash.

National aviation authorities continued to send conflicting messages to airlines and passengers on Sunday. While some isolated airports, like Frankfurt, Berlin and Warsaw, cleared the way for a handful of flights heading away from the ash cloud, most flights in northern and central Europe remained grounded.

The British transportation secretary, Andrew Adonis, ruled out any immediate change, saying flights across northern Europe “will not be safe” on Monday. But Scandinavian Airlines said it planned to operate flights Sunday night from the United States to Oslo and Stockholm.

Complicating any decisions is the continued eruption of the Icelandic volcano, Eyjafjallajokull.

As the ash cloud from the volcano has spread, it has shut down airports from the British Isles to Ukraine, disrupting the travel plans of nearly seven million travelers, according to one industry estimate. Delta Air Lines, Cathay Pacific of Hong Kong, Qantas of Australia and China Airlines of Taiwan were among those that canceled Europe-bound flights through Monday.

Since the ash cloud first appeared over European airspace Thursday, more than 63,000 flights have been canceled. Eurocontrol, the Brussels-based agency that coordinates air traffic management across the region, said that 20,000 flights, out of a regularly scheduled 24,000, were canceled on Sunday.

The interruption in service, particularly across the Atlantic, comes as the industry had just started to recover from the global recession, with business and international travel picking up.

After the spike in oil prices in 2008, several airlines went bankrupt, including Eos, an all-business-class carrier that offered flights between Kennedy International Airport in New York and Stansted Airport in London. Analysts said Europe’s legacy flag carriers, including British Airways, Lufthansa and the Air France-KLM combination, would feel the most pain from the shutdown because they have high fixed costs.

European airlines were already suffering from the slow pace of economic recovery there. Even before the latest crisis, the International Air Transport Association had projected that the industry would lose $2.8 billion this year, down from last year’s loss of $9.4 billion.

“These disruptions could not have come at a more difficult time as airlines in Europe and elsewhere struggle,” said Steve Lott, a spokesman for the industry group. “The big wild card is, how long does this last and how long before we can get flights back in the air?”

In a conference call Sunday with Eurocontrol, one airline representative sharply chastised national civil aviation authorities for being inconsistent in applying flight restrictions and stressed that the flight bans were creating “a serious economic issue for us.”

“I understand the requirement for safety,” said Dale Moss, the chief executive of OpenSkies, the Paris-based, all-business-class subsidiary of British Airways. “But the level of frustration is painfully high” for both airlines and passengers, he said. “We need some scientific data quick, so that we can start projecting and putting contingency plans in place.”

KLM received permission from authorities to fly three cargo planes to Asia on Sunday night. The airline’s chief executive, Peter Hartman, suggested that passenger flights would be safe “with the exception of an area in the north between Iceland and Russia,” according to Dutch news services.

The events in Europe have also had an economic effect on American carriers. The four-day closing of European airports so far has meant lost revenue of $80 million for United States carriers that fly the lucrative trans-Atlantic route, said Michael Boyd, an aviation consultant. It also meant a loss of connecting passengers, who often fly from Europe to the United States, and then on to another domestic destination.

“We’ve never seen such a wall across the Atlantic,” said Mr. Boyd. “If you are heavily dependent on traffic to Europe, you are in a world of hurt right now.”

Even if flights resumed this week, it could take days for the situation to return to normal, said Darin Lee, an airline specialist at LECG, an economic consulting firm in Cambridge, Mass.

“Reaccommodating several days worth of passengers can take quite a while, since there just aren’t as many empty seats on flights as there used to be,” Mr. Lee said. “Some passengers could face very long waits before being able to take their journey, and for many passengers, it simply won’t be practical to reschedule.”

While much of Europe’s airspace remained closed, the airport in Reykjavik, Iceland’s capital, was still open for business since southern winds were pushing ash away from the small rocky island in the North Atlantic.

The shutdown of Heathrow airport, the world’s busiest, also allowed authorities for the first time to inspect runways on foot in broad daylight instead of in the middle of the night.