BEIRUT: Militant group the Islamic state of Iraq and Greater Syria (ISIS) seized a Syrian gas field and killed at least 23 people Thursday in one of the bloodiest clashes between the Al-Qaeda offshoot and President Bashar Assad's forces, an activist group said.
ISIS has been making rapid gains in Syria in recent weeks, mostly by seizing territory from rival rebel groups using weaponry brought in from Iraq, where last month it managed to take large areas from government forces.
Activists say the Syrian air force has in recent weeks stepped up attacks on positions held by ISIS.
The group launched an attack against the Shaar gas field east of Homs Thursday morning, killing at least 23 of the men guarding it in a "wide assault" from several directions, the Britain-based Syrian Observatory for Human Rights said.
The Observatory, which monitors violence in Syria through a network of sources in the country, quoted "trusted sources" as saying 340 other guards, government forces and militia members loyal to Assad had been taken prisoner, wounded or killed.
"Since the beginning of the year there have been clashes between ISIS and the regime in some areas, but these are the largest," the Observatory's director Rami Abdul-Rahman said.
Previous large clashes between government forces and ISIS had usually involved other rebel groups, he said. The two sides have also skirmished continuously in other areas but casualties had been relatively light, Abdul-Rahman said.
It was not immediately possible to verify the report. Syrian state media made no mention of the attack.
About 30 had managed to escape to the nearby Hajjar field, the Observatory report added.
ISIS 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.
ISIS 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 civil war after a government crackdown.