The Islamic State group has killed more than 150 troops captured in recent fighting for a string of military bases in Syria, activists say.
Militants reportedly shot some and slashed others with knives in the past 24 hours in the latest mass killing attributed to the extremists.
In southern Syria meanwhile, gunmen detained 43 UN peacekeepers during fighting on the Syrian side of the Golan Heights, the United Nations said. It added that another 81 peacekeepers were trapped in the area by the heavy clashes between rebels and Syrian troops.
The killing of government troops, combined with photos of dusty, terrified conscripts under militant guard in the desert, underscored how the extremist group uses violence - and images of it - to instil fear in its opponents as it seeks to expand the proto-state it has carved out in Syria and neighbouring Iraq.
The Britain-based Syrian Observatory for Human Rights said that many of the soldiers killed were rounded up yesterday in the arid countryside near the Tabqa airfield, three days after Islamic State fighters seized the base.
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.
The Observatory said around 120 captive government troops from Tabqa were killed near the base. Islamic State fighters killed at least another 40 soldiers, most of whom were taken prisoner in recent fighting for other bases in the area, in the Hamrat region near Raqqa city, the group's stronghold.
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.
A video that emerged online purportedly showed Islamic State fighters escorting nearly 200 men, stripped down to their underwear and barefoot, through the desert. Another video posted online later showed more than 150 men, also in their underwear, lying motionless - apparently dead - in a row in the sand.
In its rise to prominence over the past year, the extremist group has frequently published graphic photos and videos of everything from beheadings to mass killings.
In Iraq, for example, the group killed nearly 200 men - most of them Iraqi soldiers - in late June near the northern city of Tikrit, human rights groups and Iraqi officials say.
It published photos online showing dozens of men dressed in civilian clothes lined up or lying face down as militants aimed rifles at their backs. A final set of photos showed their bloodied bodies.
Such killings have not been limited to Iraq. Earlier this month, Islamic State fighters shot and beheaded hundreds of tribesmen in eastern Syria who had risen up against the group.
A UN commission accused the Islamic State group yesterday of committing crimes against humanity in Syria, echoing UN accusations against the group in Iraq.
The Islamic State group's surge is one aspect of Syria's multi-layered civil war, a bloody conflict that has killed more than 190,000 people and destabilised the region.