BEIRUT (AFP) - Syrian rebels, including from the Al-Qaeda-affiliated Al-Nusra Front, have seized a key eastern oil field, the Syrian Observatory for Human Rights said on Saturday.

"Fighters from Al-Nusra and other groups have taken the Omar oil field in Deir Ezzor province after clashes overnight," Observatory director Rami Abdel Rahman told AFP.

He said government troops had withdrawn from the field, one of the biggest in Syria.

The army already lost the field once, in November 2012, but then recaptured it.

Video posted online by opposition activists showed fighters milling around an entrance to the field, as others drove a tank said to have been captured from regime troops after they withdrew.

Speaking to the camera, one activist said fighters had captured seven tanks.

The opposition took its first oil field last year. Since then, groups operating in rebel-held territory have began producing oil for sale on the black market.

Syria produced some 420,000 of barrels of oil a day before the United States and the European Union banned the import of Syrian petroleum and petroleum products in late 2011 to put pressure on the regime.

Oil exports plunged from 13,500 tonnes in the fourth quarter of 2011 to 7,500 tonnes in the first quarter of 2012. Europe had previously bought 95 per cent of Syrian oil, generating a third of the country's revenue.

Crude output now stands at barely 10 per cent of its pre-uprising levels as a result of the sanctions and rebel capture of key oilfields that has hit even production for domestic use.