BEIRUT (AFP) - Islamic State jihadists posted grisly video footage Thursday of scores of bodies heaped in the desert they boasted were those of Syrian soldiers they captured and executed in their latest advance.

The dead were dressed only in their underwear, stripped of any uniform, after what IS said was a mass execution of government troops following their capture of a key northern air base on Sunday.

Britain-based monitoring group the Syrian Observatory for Human Rights said that at least 160 soldiers had been executed at three separate locations around Raqa province, among some 500 who had made a desperate bid to escape after the loss of the Tabqa base.

The footage posted by IS showed a close-up of some 20 bodies, but then panned out to show scores more.

The lack of any signs of blood in the sand at the scene suggested that the dead might have been executed elsewhere and their bodies then moved.

Other shots showed men barefoot and dressed only in their underwear walking in line with their hands on their head in surrender, escorted by jihadist gunmen.

One held up the black flag of the jihadists. Others chanted: "Islamic State forever."

Observatory director Rami Abdel Rahman said the jihadists captured and killed more than 160 soldiers who had been desperately trying to reach government-held territory in the Orontes valley to the west after their weekend defeat.

Tabqa was the last position in Raqa province to fall to the jihadists, who now control a vast swathe of northeastern Syria and neighbouring Iraq.

The jihadists have repeatedly posted often gruesome videos, both as a warning to those joining up to the Syrian or Iraqi armies and as a propaganda tool to recruit volunteers from the wider Islamic world.