An attack on a military bus in northern Syria has killed 13 people, most of them Assad regime's troops, the regime's Defense Ministry said on Monday morning.

A ministry statement said that 11 government troops and two civilians were killed in what it said was a terrorist attack on a civilian transit bus. It said another three soldiers were wounded.

Syrian state news agency SANA said the attack took place in the Jabal al-Bishri region of the vast province of Raqqa, which was once controlled by the Daesh terrorist group.

The report did not say whether the bus was ambushed and attacked with machinegun fire, or whether it was hit by a missile or a roadside bomb.

The Syrian Observatory for Human Rights said the ambush had been carried out by Daesh sleeper cells that launch hit-and-run attacks in the country's desert areas.

There was no immediate claim of responsibility for the attack.

Syria's 11-year-old conflict has carved the country into various zones of control, with regime troops and allied fighters controlling the most territory.

A patch of northwestern territory is held by Turkish-backed Syrian opposition groups and more hardline groups while the northeast is held by the U.S.-backed YPG, the Syrian offshoot of the PKK terrorist organization.

One of the deadliest bus ambushes was in December 2020, when 28 people were killed in an attack on a main highway in Syria's eastern Deir el-Zour province.