BEIRUT (AP) -- Fighters from the extremist Islamic State group stormed parts of a besieged army base in northern Syria on Thursday, setting off clashes that left dozens killed or wounded on both sides, activists said.
The assault, which began shortly after midnight, came a week after Islamic State fighters captured a gas field in the central province of Homs, an attack that left more than 200 people dead.
Fighters from the Islamic State group have in the past few weeks seized a huge chunk of territory straddling the Iraq-Syria border, where they declared a self-styled caliphate. They also have captured much of Syria's oil-rich eastern province of Deir el-Zour, which borders Iraq.
The Britain-based Syrian Observatory for Human Rights said the assault began with two suicide car bomb attacks. It added that Syrian army helicopters attacked jihadi positions around the base, known as Division 17, in the province of Raqqa.
The Observatory said the fighting left 35 Islamic State fighters dead and dozens of government forces dead or wounded, including six soldiers who were beheaded. It said both sides exchanged mortar and artillery fire.
The Local Coordination Committees, another activist group, said the air force conducted 12 raids around Division 17 and the nearby provincial capital of Raqqa, which is controlled by the Islamic State group. It said army helicopters were dropping barrel bombs around the base.
The base has been under siege by rebels since much of Raqqa province fell to opposition fighters last year. The Islamic State group, which captured much of Raqqa earlier this year, has tried to capture the base on several occasions.
The Syrian conflict has killed at least 170,000 people, a third of them civilians, and displaced some 9 million, a third of the country's pre-war population.