The slayings were the latest massacre attributed to the extremist group, which has terrorized rivals and civilians alike with widely publicized brutality in Syria and Iraq as it seeks to expand a proto-state it has carved out on both sides of the border.
In southern Syria, meanwhile, gunmen detained 43 U.N. 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 heavy clashes between rebels and Syrian troops.
The mass killing of Syrian soldiers is part of a stepped up campaign by Islamic State militants targeting President Bashar Assad's forces. Until recently, the group had been focused on eliminating rivals among the rebels fighting to topple him, systematically routing Western-backed opposition fighters and other Islamic factions from towns and villages in northern and eastern Syria as it expands.
More recently, the jihadists have turned their attention to Assad's forces, seizing a series of military bases in northeastern Raqqa province. In the process, they have killed hundreds of pro-government forces, beheading some and later displaying their severed heads on poles and fences and posting the pictures online.
Most of the dozens killed over the past 24 hours were rounded up Wednesday 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 front lines after it fell to the jihadi fighters.
The Britain-based Syrian Observatory for Human Rights said around 120 captive government troops from Tabqa were killed near the base. Islamic State fighters also killed at least another 40 soldiers, most of whom were taken prisoner in recent fighting at other bases in the Hamrat region near Raqqa city, the group's stronghold.
A statement posted online and circulated on Twitter 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, some with their hands behind their heads. The photos could not immediately be verified, but correspond to other AP reporting.
The group also posted a video showing Islamic State fighters forcing the barefoot men to march through the desert. Another video later showed dozens of bodies piled in the desert, alongside others lying motionless — apparently dead — in a row in the sand.
While the videos could not be independently confirmed by the AP, they appeared to illustrate the claims made online by the Islamic State group and Syrian opposition activists about the mass killing.
There was no immediate comment from the Syrian government, which has been tight-lipped on the massive death toll incurred by its soldiers at the hands of Islamic State fighters in the past two months.
In its rise to prominence over the past year, the extremist group has frequently published graphic photos and videos of everything from bombings and beheadings to mass killings and images of jihadis taunting and humiliating terrified troops or other opponents.
In Iraq, 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 clothing lying face down as militants aimed rifles at their backs. A final set of photos showed their bloodied bodies.
Earlier this month, Islamic State fighters shot hundreds of tribesmen in eastern Syria who had risen up against the group. Some were beheaded. Last week, they posted a video showing the beheading of U.S. journalist James Foley.
The release of gruesome photos and videos documenting the slayings underscores how the extremist group uses violence — and images of violence — to instill fear in its opponents at home and win recruits abroad.
In Washington, President Barack Obama played down the prospect of imminent U.S. military action against strongholds of the Islamic State group in Syria, saying "we don't have a strategy yet" for confronting the militants.
Briefing reporters at the White House shortly before convening a meeting of his national security advisers to consider options on the issue, the president urged a regional approach that includes support from other Mideast nations.
It's time to "stop being ambivalent" about the aims of extremist groups like the Islamic State, Obama said. "They have no ideology beyond violence and chaos and the slaughter of innocent people."
A U.N. commission accused the Islamic State group Wednesday of committing crimes against humanity in Syria — echoing U.N. accusations against the group in Iraq.
The U.S. has been conducting airstrikes against Islamic State targets in Iraq, and is now considering extending its campaign to Syria, home to the group's declared capital of their self-styled caliphate.
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 destabilized the region.
The 43 U.N. peacekeepers were detained by an armed group early Thursday on the Syrian side of the Golan Heights, where fighting has raged this week between Syrian rebels and government forces.
The U.N. said another 81 peacekeepers were being "restricted to their positions" in the vicinity of Ruwaihaniyeh and Burayqa.
The office of U.N. Secretary-General Ban Ki-moon did not identify the armed group that was holding the peacekeepers. Several rebel groups operate in the Golan, while the Islamic State group has no known presence there.
"We are dealing with non-state armed actors," the spokesman for the U.N. secretary-general, Stephane Dujarric, told reporters in New York.
"We're not in a position to confirm who is holding whom. Some groups self-identified as being affiliated with al-Nusra, however, we are unable to confirm it," Dujarric said, referring to the al-Qaida-linked group, al-Nusra Front.
He said the 43 detained peacekeepers were from Fiji while 81 troops from the Philippines have had their movements restricted.
The Syrian government denounced the "kidnapping" of the U.N. peacekeepers and called for their immediate release.
The peacekeepers are part of UNDOF, the mission that has been monitoring a 1974 disengagement accord between Syria and Israel after their 1973 war. As of July, UNDOF had 1,223 troops from six countries: Fiji, India, Ireland, Nepal, Netherlands and the Philippines.
Syrian rebels briefly abducted U.N. peacekeepers twice in 2013 before eventually releasing them unharmed.
Heavy fighting has engulfed the Syrian side of the Golan since Wednesday, when rebels captured a crossing on the disputed frontier with Israel. A rebel spokesman said the militants are focused on fighting Assad, and pose no threat to Israel.
On Thursday, government warplanes targeted several rebel positions in the area, including in the village of Jaba, Syrian activists said.
White plumes of smoke set off by exploding mortar rounds could be seen Thursday from the Israeli side of the Golan as the sound of small arms fire echoed in the background.
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