Rebel fighters seized the town of Mayadeen in eastern Syria on Thursday, a monitoring group said, adding that a large area on the Syrian-Iraqi border is now under control of insurgents.

"The area east of the city of Deir Ezzor, on the Iraqi border, is now the largest area in the whole country that is out of army control," said Syrian Observatory for Human Rights director Rami Abdel Rahman.

The oil and gas-rich area of Deir Ezzor is home to Sunni Muslim tribes whose ties extend across the border into Iraq.

"In that area of Syria, there are deep tribal loyalties, and fighters in western Iraq help the rebels fight the regime," Abdel Rahman told AFP.

Troops expelled by rebels from Mayadeen were forced to withdraw to a military airport east of the city of Deir Ezzor.

Meanwhile, most oil and gas fields in the large desert province are still under regime control, said the Britain-based Observatory, which relies on a network of activists, lawyers and medics on the ground.

Deir Ezzor neighbours Iraq, and has seen intense fighting since mid-summer.

The Syrian National Council, one of the country's main opposition groups, declared Deir Ezzor a "disaster area" last month.