"Syrian regime forces and commanders of Iran-backed allied groups suddenly withdrew from Deir Ezzor city and its countryside with columns of soldiers heading towards the central Palmyra region," Rami Abdel Rahman, who heads the Syrian Observatory for Human Rights war monitor, told AFP.

Their move towards Palmyra, in central Syria, came as Islamist-led rebels who had launched a lightning offensive last week were at the gates of Homs city, to Palmyra's west.

Deir Ezzor city is home to Iranian advisers brought in after Syria's war began in 2011, as well as Iranian institutions, schools and a cultural centre.

Iran-backed paramilitaries have bolstered the fighting force of the Syrian government of President Bashar al-Assad throughout the war, triggered by the brutal suppression of a pro-democracy uprising.

Deir Ezzor province, an oil-rich region bordering Iraq, is split between Kurdish forces to the east of the Euphrates and Iran-backed Syrian government forces and their proxies to the west.

The government-held part of Deir Ezzor near the frontier with Iraq is a key hub for Iran's footmen, including Iraqi militia fighters.

The conflict since 2011 has killed more than half a million people.

Since fighting resumed on November 27 with the rebels' offensive, the Observatory said 826 people, mostly combatants but also including 111 civilians, have been killed.