Militants with the al-Qaeda breakaway group Islamic State seized control of a military base in northern Syria, beheading dozens of soldiers and displaying their headless bodies, activists said.
It is only the latest victory for the extremist group that has made rapid advances in both Iraq and Syria in recent months as it pursues its goal of creating an Islamic caliphate.
There have been sporadic skirmishes around the base, called Division 17, in Raqqa province for months, but on Wednesday Islamic State fighters launched a concerted offensive.
The fighting began when two militants blew themselves up with suicide car bombs near the base's chemistry battalion, according to the Syrian Observatory for Human Rights, a pro-opposition watchdog group based in Britain.
Within a day the militants had taken the base and begun carting off light and heavy weaponry, said Abu Muhammad, an activist with the Masar Press Agency.
"Every government soldier they capture they execute immediately," he said.
A Twitter account linked to Islamic State also published pictures of the beheaded corpses and heads of five soldiers it said belonged to the 17th division.
A separate video posted online appeared to show an Islamic State fighter filming himself entering an abandoned part of the base and tearing down images of Assad and Syrian flags.
The video, with the title "What is left of Division 17 in Raqqa," in German, shows the fighter opening a refrigerator, pulling out wine bottles and then pouring one of them on the ground outside in front of other armed men.
However, it was not immediately possible to verify the contents of the video independently.
The group, previously known as the Islamic State in Iraq and the Levant (ISIL) and as Isis, has mainly advanced in Syria by capturing land from more moderate rebel fighters. But it is now clashing more often with the Syrian military directly, and the army has responded by stepping up aerial bombing of its positions.
When the Islamic State, which grew from al-Qaeda's Iraqi branch, first entered the Syrian conflict last year, it fought alongside other rebel forces.
But soon it turned against others in the opposition, accusing them of corruption, executing their fighters and activists and pushing them out of towns and villages.
As the Islamic State has expanded into neighbouring Iraq and consolidated its hold on major parts of eastern Syria, it has launched offensives against regime targets as part of its plan to establish the Islamic caliphate in both countries and beyond.
The Islamic State now holds about a third of Syria, according to the observatory.
In New York, the head of the independent UN commission on Syrian war crimes said the Islamic State commanders were "good candidates" to be put on a confidential list of alleged war crime perpetrators.
Brazilian diplomat and scholar Paulo Sergio Pinheiro said the group had carried out public executions, crucifixions and other "gross human rights violations".
Additional reporting by Agence France-Presse, Reuters