At least 23 soldiers loyal to Syria's Bashar Assad regime were killed and 10 others injured when a bus carrying them was attacked by suspected Daesh terrorists, the Syrian Observatory for Human Rights (SOHR) war monitor said Friday.

The attack took place near the town of al-Mayadeen in the vast desert province of Deir el-Zour, which is split in control between Syrian troops, backed by Iran and Russia, and the U.S. backed YPG terrorist group.

The SOHR described it as the deadliest attack so far this year by Daesh. The group, which seized wide swathes of Syria and neighboring Iraq from 2013, has gone underground since losing its last territory in eastern Syria in 2019.

Syrian state media carried no immediate reports on the incident and there was no immediate claim of responsibility.

Attacks by Daesh sleeper cells in Syria, particularly in the vast desert zones they once controlled, have become bolder and bloodier in recent months, according to SOHR head Rami Abdel Abdelrahman.

Daesh named Abu Hafs al-Hashimi al-Quraishi as its new leader this month, for the first time confirming the death of its former head Abu al-Hassan al-Hashimi al-Qurayshi, whom Türkiye said it had killed in April.