BAGHDAD — Fighters from the Islamic State in Iraq and Syria have killed more than 150 captured soldiers in northern Syria in the last two days, a monitoring group said on Thursday. Video images posted online appeared to show the men being marched through the desert in their underwear by the extremists and then lying dead in the sand.

The mass killing of the soldiers represented a dark end to the battle for control of the Tabqa air base in Raqqa Province. The insurgents seized the base on Sunday after the deadliest fighting so far between ISIS and government forces.

The killings were reported on the same day that Syrian rebel fighters captured 43 United Nations peacekeepers near the demarcation line between Syria and the Israeli-occupied Golan Heights, after heavy fighting in the area between non-ISIS rebel fighters and government troops.

As it has stormed through northern Iraq and seized Mosul, that country’s second-largest city, ISIS has often distributed graphic images of its dead adversaries, to enlarge its reputation and terrify its enemies. But even by the group’s usual brutal standards, the video of jihadist fighters taunting humiliated, nearly naked men as they were led to their death were horrifying, sending shock waves through Syrian communities that have stood by President Bashar al-Assad through more than three years of civil war.