BEIRUT (AFP) - The Syrian regime offensive to retake a gas field in the centre of the country after jihadists killed 270 people has left 60 soldiers dead, a media report said Sunday.

Citing a military source, Al-Watan newspaper which has strong government links announced the regime had suffered the loss of "60 martyrs".

The counter-offensive was in its third day on Sunday, a security source told AFP.

"Fighting is continuing in the Shaar gas field" in Homs province, the source said, confirming that "the operation is not at an end".

The Syrian Observatory for Human Rights monitoring group, which has provided much of the information about the conflict, also said that "violent clashes were continuing in and around the gas field".

The watchdog said it had documented "the deaths of 270 people killed in the fighting or executed" by the jihadists from the Islamic State (IS) group.

"A large majority of the men killed were executed at gunpoint after being taken prisoner following the takeover of the camp," said director Rami Abdel Rahman.

"Eleven of the dead were civilian employees, while the rest were security guards and National Defence Forces members," he added.

Regime forces backed by warplanes had pressed a counter-attack Saturday around Shaar, recapturing large areas, said Abdel Rahman.

The counter-attack killed at least 40 IS militants, said the Observatory, which relies for its information on a network of activists and medics on the ground.

It described Thursday's takeover of the Shaar field as the "biggest anti-regime operation by the IS" since the jihadist group rose to prominence last year among rebel groups in the Syrian conflict.

The jihadists proclaimed an Islamic "caliphate" straddling Syria and Iraq last month and have also taken over Syria's oil-rich Deir Ezzor province.

Deir Ezzor borders Homs province as well as Iraq, where the jihadist group has spearheaded a major Sunni militant offensive that has seen large swathes of territory fall out of the Baghdad government's control.