BEIRUT (Reuters) - Islamic State fighters broke into the Deir al-Zor air base at dawn on Saturday, a monitoring group said, breaching the last remaining government stronghold in eastern Syria.
The al Qaeda offshoot Islamic State has been gradually consolidating control of the oil-producing Deir al-Zor province this year. President Bashar al-Assad's forces have held on to the local military air base and parts of the provincial capital.
The Britain-based Syrian Observatory for Human Rights said 51 of Assad's forces and 60 members of Islamic State had been killed in the three-day offensive.
"Islamic State have broken into the base and the regime is using air strikes outside it," Observatory head Rami Abdulrahman said by telephone.
Syrian state media did not mention a breach but said the army had killed "a number of terrorists" in al Jufra, an area close to the airport, on Friday. It said Syrian Air Force jets operating from the base had destroyed an Islamic State convoy.
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.
The Twitter account of the U.S. embassy in Syria said late on Friday that a U.S. air strike in Deir al-Zor had hit Islamic State targets: three vehicles, an excavator and a training camp.
It was unclear whether the attacks took place near the base.
Deir al-Zor borders territories in Iraq also controlled by Islamic State. The province's oilfields are a major source of revenue for the group.
(Reporting by Oliver Holmes; Editing by Dale Hudson)