US and British forces are increasingly playing a supporting role in the Iraqi Prime Minister Nouri al-Maliki's stalled offensive against the Mehdi Army militia. American aircraft launched air strikes in Basra yesterday and fought militiamen on the streets in Baghdad while British advisers have also been assisting Iraqi troops in Basra.

Mr Maliki retreated from his demand that militiamen hand over their weapons by yesterday and extended the deadline to 8 April. This is a tacit admission that the Iraqi army and police have failed to oust the Mehdi Army from any of its strongholds in the capital and in southern Iraq. The Iraqi army has either met stubborn resistance from Mehdi Army fighters or soldiers and police have refused to fight or changed sides. "We did not expect the fight to be this intense," said the officer from a 300-strong commando unit that has been pinned down in the Tamimiyah district in Basra, where the supporters of Muqtada al-Sadr, the leader of the Mehdi Army, have strong support.

The officer said four of his men were killed and 15 wounded in the fighting. "Some of the men told me that they did not want to go back to the fight until they have better support and more protection," he added. The Interior Ministry threatened that the men would be court-martialled for refusing to fight. Government troops arriving in Basra complain that they are being fired on by local police loyal to Mr Sadr. Members of one police unit had fist fights with their officers after they refused to join the battle.

The failure of Mr Maliki to make good his threat so far to eliminate the Mehdi Army and growing signs of dissent in army units is damaging his authority, "It is possible that Muqtada and the Mehdi Army will emerge from this crisis stronger than they were before," warned one Iraqi politician who did not want his name published.

Fears that Mr Maliki's surprise assault on the Mehdi Army is faltering without any real gains on the ground probably explains why US aircraft are dropping bombs in Basra and US armoured vehicles made an incursion into Sadr City in Baghdad. The explosion of violence over the past four days is making a mockery of George Bush's claim that America had turned the corner in Iraq.

The crisis has also presented the British Government with a dilemma. If the 4,100 British troops at the airport remain spectators to the battles in the city, critics will ask what they are doing there. But if they intervene in what is essentially a battle between two Shia factions they will be dragged back into the struggle for Basra in which nobody is likely to emerge the winner.

Mystery surrounds Mr Maliki's motive in launching an assault on the Mehdi Army after Mr Sadr renewed his six-month ceasefire last month. A likely explanation is that Mr Maliki, who has little support outside the holy city of Kerbala, was under pressure from the Islamic Supreme Council of Iraq (ISCI), his main ally, to attack the Sadrists now. The Sadrists were expected to do well against ISCI in provincial elections which are to be held in October under an agreement brokered by the US Vice-President Dick Cheney during his visit to Baghdad earlier in the month.

ISCI wanted to crush the Sadrists before the poll and this would be easier to do before the US reduces the numbers of its troops in Iraq. But, unless Mr Maliki's attack picks up steam over the next week, he will have done nothing except damage his own standing. Demonstrators have already been denouncing him as an American puppet and demanding that he go.

A measure of the anarchy in Iraq is that it is unclear who controls large swaths of the country. By one report the Mehdi Army has taken over the centre of the city of Nassariya. The Green Zone in Baghdad, the headquarters of the Iraqi government and of US political influence, is being mortared every day. One mortar round killed two guards outside the Vice-President's office in the zone. Nobody knows on whose side sections of the security services belong. The Iraqi spokesman for "the surge", which Mr Bush has lauded as successful, was kidnapped by police commandos, who shot dead his guards before abducting him. A curfew was in force in the capital yesterday.

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