Islamic State fighters executed scores of Syrian soldiers it captured when the group seized an airbase in the province of Raqqa over the weekend, Reuters reported Thursday.
A video posted on YouTube and confirmed as genuine by an Islamic State fighter showed the bodies of dozens of men lying face down wearing nothing but their underwear. They were stretched out in a line that appeared to be dozens of meters long.
A separate pile of bodies was shown nearby. Reuters could not independently verify the authenticity of the video.
The caption beneath it said the dead numbered 250. An Islamic State fighter in Raqqa told Reuters via the Internet, "Yes, we have executed them all."
The Islamic State, formerly known as the Islamic State of Iraq and the Levant, shortened its name after declaring a caliphate in parts of Iraq and Syria it captured.
In Iraq, it has been conducting large-scale massacres and systematic rape, particularly against Yazidi minorities, drawing concerns of an impending genocide if rapid action is not taken.
The group originally joined the fighting in Syria and made its presence known by torturing and murdering prisoners, among them children and teenagers, and forcing Druze men to convert to Islam or die.
In March, the group live-tweeted the amputation of a hand of a man charged with theft in the northern province of Aleppo.
Most recently the group released a video which shows the beheading of American journalist James Foley, who disappeared in November 2012 in Syria.
Islamic State controls roughly a third of Syria, mostly areas in the north and east of the country, noted Reuters. The United States has launched airstrikes on the same group over the border in Iraq and is considering doing the same in Syria.