Islamic State fighters and Syrian regime troops have clashed in fierce battles that reportedly cost hundreds of lives over the Tabqa military air base, the last stronghold of the Syrian army in Raqqa province.
The Syrian Observatory for Human Rights reported on Sunday afternoon that fighters were in control of the base in northeastern Syria but that clashes were still taking place.
It is the third military base in the area to fall to IS since last month. Islamic State (IS) is an Al-Qaeda breakaway group that has established what it calls a "caliphate" across vast swathes of territory straddling the Syria-Iraq border.
The group launched its offensive on the Tabqa base last week. After several failed efforts to breach the walls, IS fighters stormed the air base Sunday, according to the U.K.-based Observatory.
"Some of the Syrian regime troops pulled out, and now the Islamic State is in full control of Tabqa," said Observatory director Rami Abdurrahman. "This makes Raqqa province the first to fully fall out of government hands."
The SANA state news agency confirmed that the government had lost the air base, saying troops "are successfully reassembling after evacuating the airport." It said that the military was still "striking terrorist groups, inflicting heavy losses on them."
The government had made significant investments in both weapons and manpower to try to hold onto Tabqa, making its fall both a symbolic and a strategic blow.
The air base is one of the most significant government military facilities in the area, containing several warplane squadrons, helicopters, tanks, artillery and ammunition
The Observatory said at least 346 IS fighters were killed and more than 170 members of government forces had died since Tuesday in the fight over Tabqa base, making it one of the deadliest confrontations between the two groups since the start of Syria's war in 2011.
Abdurrahman said the fighters had also seized several checkpoints, and at one had hung the head of a soldier who had been killed in the fighting and decapitated.
Elsewhere, the Observatory said IS fighters were withdrawing from parts of central Homs province on Sunday.
The monitoring group said there was no official reason given for the withdrawal, but that the fighters appeared to be moving to areas under tighter Islamic State control, including Deir Ezzor province in the east.
Also on Sunday the foreign ministers of Saudi Arabia, the United Arab Emirates, Egypt, Qatar and Jordan met in the Red Sea city of Jiddah, the official Saudi news agency said without providing details.
But Egypt's Foreign Ministry said ahead of the talks that the group would discuss the security threat posed by the IS and search for ways to bring about a political solution to the Syrian crisis.