Islamic State (IS) militants appear to have brutally executed "dozens" of fleeing Syrian army soldiers, a group monitoring the violence said on Thursday.
The Syrian Observatory for Human Rights said the soldiers were captured after IS stormed and took over an air base in the northeast of the country, in the latest in a long line of brutal abuses alarming Western powers.
Chilling photos and a disturbing video were posted online appearing to show more than 100 men in their underwear being marched through the desert at gunpoint - presumably to their deaths.
Observatory director Rami Abdurrahman told the Associated Press the extremists killed up to 65 soldiers. He said some were shot to death, while others were killed with knives.
He told AFP the defeated garrison comprised 1,400 soldiers, 200 of whom were killed and 700 of whom managed to escape.
Two hundred appeared to have been caught and executed by IS as they attempted to cross the desert to government-held territory in the Orontes Valley to the west, he said. Another 500 men were on the run, he added.
A statement posted online and circulated on Twitter by supporters of the Islamic State group also claimed the extremists killed "about 200" government prisoners captured near Tabqa.
In the video, militants take photos and video on their phones as they jeer at the young men and shout "Islamic State" and "There is no going back". News agencies including the Huffington Post have been unable to confirm the contents of the video.
In images published earlier this week, IS fighters wearing balaclavas could be seen shooting dead at least seven kneeling men at the airbase.
Pictures also claim to show heat-seeking missiles, anti-aircraft guns and vast stores of ammunition captured by the jihadi fighters.
Syrian state media confirmed the attack on the base but has not reported any deaths or any army members being captured. It has said Islamic State suffered heavy losses in the battle over the base, Reuters reported.
On Wednesday, a UN commission accused the extremist group of committing crimes against humanity in Syria. The UN has accused the group of similar crimes in Iraq.
The militant group is carrying out regular amputations, lashings, public executions and mock crucifixions, according to a stark report published after the three-year investigation into the war-torn state.
The report on the Syrian Arab Republic for the UN Office for Human Rights details how "war crimes and crimes against humanity" are being inflicted on the Syrian people by rebel and ISIS extremists, and by the regime of Bashar al-Assad, which continues to deploy chemical weapons in civilian areas.
The report states: "In areas of Syria under Isis control, particularly in the north and northeast of the country, Fridays are regularly marked by executions, amputations and lashings in public squares.
"Executions in public spaces have become a common spectacle on Fridays in (IS power-base) Raqqa and in (IS)-controlled areas of Aleppo governorate... Bodies of those killed are placed on display for several days, terrorising the local population."
According to the UN report, boys as young as 15 are being beheaded, while women are lashed for "crimes", such as being inappropriately dressed, or are being stoned to death over accusations of infidelity.
The report adds that civilians, including children, are rounded up and forced to watch the acts of brutality as a warning, while many boys, some as young as 10, are being recruited to the ISIS cause for use in combat or on suicide missions.