An attack on a military bus in Syria's east killed 23 government troops and wounded more than 10, the Syrian Observatory for Human Rights (SOHR) war monitor said on Friday.
Islamic State, which operates sleeper cells in lands it once ruled, claimed responsibility for the attack in a statement on its Telegram channel.
It took place near the town of al-Mayadeen in the vast desert province of Deir Ezzor, which is split into areas controlled by Syrian troops backed by Iran and Russia, and Kurdish-led fighters backed by the United States.
The SOHR described it as the deadliest attack so far this year by Islamic State. 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.
Attacks by Islamic State 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.
Islamic State 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 Hussein al-Husseini al-Quraishi, whom Turkey said it had killed in April.
(Reporting by Clauda Tanios, Maya Gebeily, Enas Alashray and Yomna Ehab; Editing by Peter Graff and Philippa Fletcher)