Fighters from the Islamic State group have taken over an airbase in northeast Syria, capturing it from government forces after fighting that cost more than 500 lives, a monitoring group and state media have said.

The Syrian Observatory for Human Rights (SOHR), a London-based rights group, said at least 346 Islamic State fighters and more than 170 government forces had been killed since Tuesday in the fight over the Tabqa base, which was captured by fighters on Sunday.

The SOHR, which monitors violence in Syria through sources on the ground, said fighting raged inside the air base throughout Sunday.

Syria’s official news agency said the military had withdrawn from the base after pitched battles and was still carrying out strikes.

The base was the Syrian army’s last foothold in an area otherwise controlled by the self-declared jihadist Islamic State group, which has seized large areas of Syria and Iraq.

It is one of the most significant government military facilities in the area, containing several warplane squadrons, helicopters, tanks, artillery and ammunition.

In nearby Raqqa city, an Islamic State stronghold, there was celebratory gunfire and the development was announced by several mosques through their loudspeakers, a witness told the Reuters news agency.

Fighters displayed the severed heads of Syrian army soldiers in the city square, the witness reportedly said, adding that Syrian warplanes were heard over Raqqa following the air base attack.

Earlier on Sunday the Syrian air force had bombed areas around the base.

The Observatory said that the base was strewn with bodies of “dozens” of soldiers.

Army forces ‘regrouping’

“After heavy fighting by the forces defending the Tabqa airport, our forces implemented a regrouping operation after the evacuation of the airport,” Syrian state television said on Sunday.

It added that army troops were launching “precision strikes” against “terrorist groups in the area, inflicting heavy losses”.

Syrian state media gave no figure for the number of people killed in the clashes.

Regime forces had repelled three previous attacks on the base in the previous week.

The Syrian army sent reinforcements to the base overnight on Friday to fight the Islamic State, which controls roughly a third of northern and eastern Syria, the Reuters news agency said.

The Islamic State also trapped around 150 retreating Syrian soldiers in an area near the base and was believed to be holding them captive, the Observatory said.

Many Islamic State fighters died after Syrian warplanes bombarded the area, the rights group added.

The Islamic State has taken three Syrian military bases in the area in recent weeks, boosted by arms seized in Iraq.

Tabqa is the last army stronghold in the Raqqa, after fighters captured Brigade 93 and Division 17 in the northern province, killing dozens of soldiers, many of whom were beheaded.

Raqqa province has become the stronghold of the Islamic State, which controls the provincial capital and has declared an Islamic “caliphate” in territory it holds in Syria and Iraq.

The group initially fought alongside Syrian opposition groups, but its abuses sparked a backlash from rebels who pushed it out of parts of northern Syria. Al Jazeera