Fighters from an extremist Islamist faction killed more than 160 government soldiers captured over the last month in the eastern Syria province of Raqqa, according to both the group's supporters and opposition activists.
Backers of the Islamic State, the Al Qaeda splinter group now fighting in both Syria and Iraq, posted high-resolution images and videos on Twitter and YouTube purporting to show the group's fighters corralling a column of bedraggled soldiers stripped to their underwear to a mass grave.
According to the images, the soldiers were then killed by machine gun fire or had their throats slashed with swords and knives.
Many of the corpses in the images showed signs of mutilation and torture. Some, including officers in the Syrian army and air force, apparently were beheaded after death. The group's supporters also uploaded images of heads placed in a public square in the city of Raqqa.
Although the images could not be independently verified, the pro-opposition Syrian Observatory for Human Rights, with a network of activists on the ground, said in a statement that more than 160 soldiers were executed in the last 24 hours by the Islamic State. Islamic State supporters put the number at over 250.
Most of the soldiers had been taken prisoner Sunday when Islamic State fighters seized the last government stronghold in the area, Tabqa Military Airbase, about 30 miles southwest of the city of Raqqa. Another 40 soldiers were captured during the faction's takeover of two other bases in the area over the last two months, the Observatory said.
Pro-government sources, who had said the Tabqa base was evacuated before being taken over by the Islamic State, confirmed that the soldiers killed were from the Syrian military but disputed they had been taken from the base.
The report of killings came a day after the United Nations published a scathing report singling out the Islamic State for "gross atrocities" including "torture, murder, acts tantamount to enforced disappearance, and forcible displacement as part of an attack on the civilian population in Aleppo and Ar Raqqah governorates, amounting to crimes against humanity."
The grisly images are the latest example of the Islamic State's propaganda efforts. Since the faction seized a large swath of northern and western Iraq this year, it has ramped up its presence on social media with high-quality images, slickly produced promotional videos, and even a Twitter application for its supporters to quickly disseminate the group's tweets.
Aside from publicizing the latest killings, Islamic State sources on Twitter also displayed images purporting to show some of the military hardware the group had commandeered Wednesday, including tanks, artillery, rocket launchers and even a plane.
Elsewhere in Syria, rebels detained 43 U.N. peacekeepers on Thursday near the Golan Heights line separating Israel and Syria, according to a statement released by the world body. Another 81 soldiers with the international force in a nearby area had their movements restricted.
"The United Nations is making every effort to secure the release of the detained peacekeepers and to restore the full freedom of movement of the force throughout its area of operation," the statement said.
On Wednesday, rebels wrested control from Syrian forces of the Quneitra crossing into the Israeli-controlled Golan Heights, consolidating the opposition's hold of the border areas.
The clashes continued Thursday, leading to the killing of a prominent rebel commander from the Syrian Revolutionaries Front, the first rebel brigade to receive U.S.-made TOW missiles in southern Syria.