At least 19 Syrian soldiers and militiamen were killed when Islamic State attacked one of the government's last remaining strongholds in the country's east, a monitoring group and a media activist said on Thursday.
The al Qaeda offshoot Islamic State has been gradually consolidating control of Syria's oil-producing Deir al-Zor province this year, but President Bashar al-Assad's forces have held on to several areas including the local military air base.
Islamic State launched its attack on the base on Wednesday night after detonating a suicide car bomb at a nearby building where government fighters congregated, said the Britain-based Syrian Observatory for Human Rights. Islamic State fighters shelled the air base and other areas under government control.
The Observatory, which says it monitors Syria's war through a network of sources on all sides, said at least 19 government soldiers and loyalist militiamen and seven Islamic State fighters were killed. It said Islamic State captured heavy weapons including two tanks and an armoured vehicle.
The media activist in Deir al-Zor province, who asked not to be named, said Islamic State had attacked the base from three directions and also launched an attack on parts of Deir al-Zor city controlled by the government.
There was no mention of the fighting on the Syrian state news agency SANA, which on Wednesday said government forces had inflicted heavy losses on Islamic State in Deir al-Zor city, destroying a car bomb driven by a Libyan suicide attacker.
Islamic State has been under pressure from U.S. air strikes in Syria since September, but that has not stopped it from launching attacks on Assad's forces and other targets to expel government forces and rival rebels.
Assad, who is shunned by the United States as part of the problem, said in an interview with a French magazine the U.S.-led air strikes were not working.
"The coalition has not struck since the attack on the air base started," the activist said. "The last coalition air raid was about a week ago."
Deir al-Zor province borders territories in Iraq also controlled by Islamic State. The province's oilfields are a major source of revenue for the group.