BEIRUT, Lebanon — A group of Syrian rebel brigades, including an affiliate of Al Qaeda, seized a large oil and gas field from government forces on Saturday, opposition activists said, further depriving the government of President Bashar al-Assad of the resources it needs to remain solvent.

Videos posted online showed scores of black-clad rebels walking through a large arch over an entrance to the Omar oil field, rummaging through its buildings and standing atop tanks.

The Syrian Observatory for Human Rights, a Britain-based opposition group that monitors the war, said a number of rebel brigades seized the area after an overnight battle and the withdrawal of government troops. Among the groups that participated were the Islam Army, which was formed east of Damascus, the Syrian capital, and the Nusra Front, which is affiliated with Al Qaeda.

Two and a half years of civil war in Syria and strict international sanctions have battered the country’s oil sector, once an important source of government revenue.