At least 13 fighters loyal to the Syrian regime were killed Saturday in an ISIS ambush in the country’s east, a war monitor said.

The attack by the extremists killed “at least 13 members of a local pro-regime group and wounded others”, the Syrian Observatory for Human Rights said.

It took place in the Masrib area, in the west of Deir Ezzor province, while the fighters were conducting a sweep of the area, said the Observatory, which relies on a network of sources inside Syria.

Observatory chief Rami Abdel Rahman told AFP it was the deadliest attack on pro-government forces in five months.

ISIS continues to conduct attacks on government forces from hideouts in the vast Syrian Desert which stretches all the way from the Damascus outskirts to the Iraqi border.

Abdel Rahman said the militant group has recently increased its activity both in government-controlled areas and in territory controlled by the Kurdish-led Syrian Democratic Forces.

The Observatory said that 1,593 pro-government fighters and 153 allied pro-Iranian fighters had been killed in ISIS attacks since late March 2019.

It said 1,081 ISIS militants had been killed over the same period.

Earlier Saturday, the Observatory said that three Iraqis, including a woman, were killed in violence inside the Al-Hol camp in northeastern Syria that houses relatives of suspected militants.

The overcrowded camp is under the control of the Kurdish administration that runs the region but violence, mostly perpetrated by ISIS, is frequent.