ISIL has been making rapid gains by seizing territory from rival militant groups using weaponry brought in from Iraq, where last month it managed to take large areas from army forces.

The group launched an attack against the Shaar gas field east of Homs on Thursday morning, killing at least 23 of the men guarding it in a "wide assault" from several directions, the so-called Syrian Observatory for Human Rights said.

The Observatory, which monitors violence in Syria through a network of spies in the country, quoted "trusted sources" as saying 340 other guards, army forces and pro-government civilians had been taken prisoner, wounded or killed.

It was not immediately possible to verify the report while Syrian state media made no mention of the attack.

About 30 had managed to escape to the nearby Hajjar field, the Observatory report added.

ISIL has previously taken control of oil fields in Iraq as well as in Syria's eastern Deir al-Zor province. The group was once the Iraqi affiliate of Al-Qaeda, but Al-Qaeda disowned it in February after tensions mounted over its expansion into Syria.

ISIL has declared a "caliphate" in the areas where it operates in Iraq and Syria, which include the Syrian city of Raqqa as well as Iraq's Mosul.

The Observatory says more than 170,000 people have been killed in Syria's conflict, which started as a peaceful protest movement in 2011 but descended into a multifaceted war after intervention from foreign countries.

SHI/SHI