Fighters from the Islamic State group have killed at least 50 Syrian soldiers in an ambush in Syria’s northeast, summarily executing most of them after their capture, according to a monitoring group.

The reported deaths on Friday in Raqqa, a stronghold of the Islamic State group, came a day after the self-declared jihadist group launched an assault on the army's 17th Division.

The fighting has left 69 dead on the regime side, including the 50 soldiers, and killed 28 fighters, the UK-based Syrian Observatory for Human Rights reported said.

Al Jazeera cannot independently verify the information because of restrictions on reporting from inside Syria.

Media close to the regime of President Bashar al-Assad remained silent on the deaths in Raqqa.

"Some of them were killed in fighting but most of them were beheaded," Rami Abdel Rahman, director of the Syrian Observatory, told the AFP news agency.

The Islamic State group claimed the toll was as high as 75.

A Twitter account linked to Islamic State also published pictures of the beheaded corpses and heads of five soldiers killed in Raqqa, saying they belonged to the 17th division.

The assault was a rare direct confrontation of this magnitude between the Islamic State fighters and Syrian government forces, with the group intent on "cleansing" the territories it controls of the regime presence, the Syrian Observatory said.

The Islamic State group, which proclaimed a "caliphate" in an area spanning the Syrian and Iraqi borders in June, controls swathes of land in both countries and is seeking to extend its power and influence.

The group, previously known as the Islamic State in Iraq and the Levant, has mainly advanced in Syria by capturing land from more moderate rebel fighters.

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, fighters from the 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 activists.