An Islamic State militant fires his weapon in celebration after Islamic State militants took over Tabqa air base, in Raqqa city. Photo: Reuters

Islamic State fighters executed scores of Syrian soldiers captured when the militants seized an airbase in the province of Raqqa at the weekend, according to a video posted on You Tube yesterday.

The video, confirmed as genuine by an Islamic State fighter, showed the bodies of dozens of men lying face down wearing nothing but their underwear. They were stretched out in a line that appeared to be dozens of metres long.

A separate pile of bodies was shown nearby. Reuters could not independently verify the authenticity of the video.

The caption beneath it said the dead numbered 250. An Islamic State fighter in Raqqa told Reuters via the internet: “Yes, we have executed them all.”

The Syrian Observatory for Human Rights, which monitors violence in the war, put the death toll at more than 120. Islamic State, a radical offshoot of al-Qaeda, stormed Tabqa airbase on Sunday after days of clashes with the army and said it had captured and killed soldiers and officers in one of the bloodiest confrontations yet between the two sides.

The capture of Tabqa, the Syrian army’s last foothold in that area, and apparent parading and killing of large numbers of its soldiers shows how Islamic State has cemented its grip on the north of the country.

The video begins by showing the captives apparently being marched in the desert with their hands behind their heads and watched by armed men. An Islamic State fighter repeatedly shouts out “Islamic State”, to which the men reply “It shall remain”.

Islamic State controls roughly a third of Syria, mostly areas in the north and east of the country.

The United States has launched airstrikes on the same group over the border in Iraq and is considering doing the same in Syria.

Video caption states the dead Syrian soldiers numbered 250

The Syrian government, which is shunned by the West, has presented itself as a partner in a war on Islamist extremists. But Washington, which has built its Syria policy on Assad leaving power, says he is part of the problem. French President Francois Hollande said yesterday Assad was no ally in the fight against Islamic State.

Syrian warplanes yesterday hit Islamic State targets in the eastern province of Deir al-Zor, another of its strongholds, in an air strike that killed some of the group’s commanders, the Syrian Observatory for Human Rights said.

The Observatory said the planes struck a building used as an Islamic State headquarters during a meeting of its commanders. Syrian state TV reported that the army “eliminated more than 10 terrorists” in an attack east of Deir al-Zor military airport, including two men it named as Islamic State leaders in the province.

Syrian state TV reported on Sunday that its troops had withdrawn from the base and regrouped but it has not reported any army deaths or captures. It has said Islamic State suffered heavy losses in the battle over the base.

Another video posted online appeared to show at least one Syrian soldier being interrogated before a group of other captured men in their underwear, as voices off camera shout sectarian insults.

The soldier identifies himself as an officer and says he is from the Alawite sect, an offshoot of Shi’ite Islam, like Assad. Islamic State members are Sunni Muslims.

The interrogator shouts insults at the soldier and asks why the soldier had been fighting on behalf of Assad and did not defect . The man replies that he would have just been sent back to the army.

“They would have sent you right back to the army? And we’re going to send you right back to hell: by slaughter,” the interrogator says.