REBEL fighters have seized the town of Mayadeen in eastern Syria, a monitoring group says, 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 on Thursday.

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.

Meanwhile, the toll from the violence across Syria has reached at least 40,000 since the outbreak of the anti-regime revolt in March last year, the Syrian Observatory for Human Rights says.

"At least 28,026 civilians, 1379 defectors, 10,150 soldiers and 574 unidentified people have been killed in Syria in the past 20 months," said Observatory director Rami Abdel Rahman.

Syrian state television on Thursday reported government troops had killed dozens of what it termed as "terrorists" near the capital Damascus.

Daraya, on the outskirts of Damascus, has been the scene of some of the worst violence in Syria's 21-month conflict.

"The Syrian army managed in an operation that lasted two days in the area of Daraya to eliminate a terrorist group and liberate civilians who had been used by terrorists as human shields," reported the broadcaster, referring to rebels fighting to oust the regime of President Bashar al-Assad.

Mortar shells were on Thursday fired at a government facility and a private building in the district of Mezze in Damascus, said the Syrian Observatory for Human Rights.

It gave no casualty figure.

Elsewhere, rebel fighters have taken control of a government artillery base in the eastern province of Deir al-Zour and killed at least six government soldiers, reported opposition activists.

Originally published as Rebels seize strategic Syrian town