DUBAI/LONDON: Three Iranian tankers are returning to Syria for the first time since April, according to shipping data and sources, in a sign Syria has succeeded in negotiating fresh supplies of badly needed diesel from their allies.

Diesel, or gasoil as it is known in the industry, is used to power heavy vehicles including army tanks. Although Syria is an oil producer, it lacks the means to produce gasoil, and relies heavily on imports.

Shortages are crippling Syria's economy as tighter EU sanctions forced Syria's usual suppliers to halt deliveries in late March. Last month Syrian officials said they were working to secure fuel supplies from Algeria and Iran.

The first tanker, the Amin, is due to reach the Syrian port of Banias late on Tuesday, while a second vessel with a delivery of gasoil, the Alvan, is due to arrive later this week, according to ship tracking data and shipping sources.

"The Iranian tanker M/T Alvan is expected to call at Banias to discharge gasoil and load gasoline," said a shipping source who asked not to be named.

Syria's refineries are set up to process the country's light crude oil mainly into lighter products such as naphtha and gasoline, leaving it short of diesel.

The Alvan, was passing through Egypt's Suez Canal on Tuesday, according to shiptracking data.

The draft of the Amin suggests it is nearly empty and so likely picking up Syrian oil.

The vessel, which is owned by a subsidiary of the U.N.-sanctioned Islamic Republic of Iran Shipping Lines (IRISL), last visited Syria in April to pick up Syrian crude.

It returned to Iran's Bandar Abbas where it dropped anchor and it was unclear who bought the vessel's 140,000 tonne cargo of Syrian oil.

The MT Tour - another UN-sanctioned IRISL tanker that picked up a cargo of sanctioned Syrian oil earlier this year and took it to Iran - also appears to be nearly empty.

It was last shown by AIS Live ship tracking data sailing towards the Red Sea on Tuesday afternoon.

Iran has being trying to help its close ally Syria by supplying President Bashar al-Assad's government with fuel it needs while picking up crude that Damascus struggles to sell because of sanctions over its violent suppression of dissent.

The MT Tour loaded with Syrian crude in March and set sail for east Asia, was initially bound for Singapore, but diverted to Iran - which has enough difficulty finding buyers for its own sanctioned crude. The Amin also shipped crude from Syria to Iran earlier this year.

Two other tankers owned by National Iranian Tanker Company were also sailing in the Red Sea towards Suez but appear headed for the unloading terminal for the Sumed pipeline in Egypt.