Fighters from al Qaeda offshoot Islamic State killed at least 50 Syrian army soldiers and took over parts of their base on Friday outside the northern city of Raqqa, as the radicals escalated their attacks on government forces, a monitoring group said.
Fighters from the ultra-hardline group captured and killed at least 50 members of the army's 17th Division after ambushing them outside Raqqa when they withdrew from the area where they were based, the Britain-based Syrian Observatory for Human Rights said.
Raqqa is already a stronghold of Islamic State, which has advanced in Syria and taken over swaths of territory in neighboring Iraq in what it has described as a bid to establish an Islamic caliphate.
Since its lightning advance in Iraq last month, the group has confronted government forces in Syria more frequently whilst continuing to attack rival rebel groups fighting to oust President Bashar al-Assad.
Raqqa is the only Syrian provincial capital to have fallen completely outside Assad's control and Islamic State has paraded military hardware through its streets.
A Twitter account linked to Islamic State also published pictures of the beheaded corpses and heads of five soldiers killed in Raqqa province, saying they belonged to the 17th division.
A separate video posted online on Friday 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 ground outside in front of other armed men.
However, it was not immediately possible to verify the contents of the video independently.
The Islamic State draws its strength in Syria from foreign fighters and members sometimes post videos filmed in a mixture of Arabic and other languages.
The group, previously known as the Islamic State in Iraq and the Levant (ISIL), 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 bombings on its positions.
Last week, Islamic State killed 270 soldiers, guards and staff when it captured a gas field in central Syria, in one of the deadliest clashes between the group and government forces, according to the Observatory, an anti-Assad monitoring group that tracks the violence using its sources on the ground.
Syrian soldiers also fought Islamic State militants outside a government-controlled army airport in Deir al-Dor province in the east of the country last Friday.