DAMASCUS, Syria (AP) — Ten people were killed and one person injured in an attack on a bus carrying workers in an oil field in Syria’s government-held areas, Syria’s state news agency reported Thursday.

No details were offered of the nature of the attack or who was behind it.

The news agency SANA called it a “terrorist attack” and said the workers were employers of the government-controlled Kharata oil field in Deir el-Zour province.

The Britain-based Syrian Observatory for Human Rights, which monitors Syria’s war decade-old war, said an explosive device detonated killing 10 of the oil-field workers on the bus. It was not clear if the device was laid on the side of the road or lobbed at the bus.

Islamic State militants have been active in the desert area in east and central Syria despite losing territorial control in the country since 2019. They have previously carried out attacks in the oil-rich Deir el-Zour, where the militants last held territory.

The militants were defeated in a military campaign by Kurdish forces in collaboration with US-led coalition against IS in March 2019.

Since then, thousands of militants are believed to be hiding in the deserts of Syria and Iraq, carrying out a low-level insurgency, including attacks on government and Kurdish forces as well as military posts and oil infrastructure.