BEIRUT (AP) — The Islamic State group has killed dozens of Syrian soldiers it captured after overrunning a military base in northeastern Syria, shooting some and using knives on others in the latest brutal mass killing attributed to the extremists, activists said Thursday.

The Britain-based Syrian Observatory for Human Rights said the militants rounded up the soldiers in the arid countryside on Wednesday near the Tabqa airfield, three days after seizing the base in heavy fighting. The government troops were among a large group of soldiers from the base who were stuck behind the front lines after the airfield fell to the jihadi fighters.

Observatory director Rami Abdurrahman said the extremists killed up to 65 soldiers. He said some were shot to death, while others were killed with knives.

A statement posted online and circulated on Twitter by supporters of the Islamic State group claimed the extremists killed "about 200" government prisoners captured near Tabqa.

It also showed photographs of what it said were the prisoners: young men stripped down to their underwear marching in the desert. The photos could not immediately be verified, but correspond to other AP reporting.

The photos and reported mass killing underscored how the group uses violence — and images of violence — to terrorize its opponents, as it sweeps further into Syria and Iraq. The group has declared an Islamic state, or caliphate, in the territories it controls straddling the border.

On Wednesday, a U.N. commission accused the extremist group of committing crimes against humanity in Syria. The U.N. has accused the group of similar crimes in Iraq.

In southern Syria, meanwhile, government warplanes targeted rebels near the country's frontier with Israel in the Golan Heights, a day after opposition fighters captured a crossing point on the disputed border.

The Syrian airstrikes hit several areas near the frontier in Quneitra province, including the village of Jaba, said the Observatory, which relies on a network of activists inside Syria. The Local Coordination Committees, another activist group, also reported the air raids.

The Observatory said heavy clashes were raging between the rebels and the Syrian military in Jaba and the surrounding countryside.

White plumes of smoke set off by exploding mortar rounds could be seen on Thursday from the Israeli side of the Golan. The sound of small arms fire could be heard echoing in the background.

An array of Syrian rebel groups, including the al-Qaida-linked Nusra Front, captured the frontier crossing on Wednesday. A rebel spokesman said the opposition is focused on fighting President Bashar Assad, and poses no threat to Israel.