An attack on a military bus in Syria's east killed 23 pro-Assad soldiers and wounded more than 10, the Syrian Observatory for Human Rights (SOHR) war monitor said Friday, blaming the Islamic State group that operates sleeper cells in lands it once ruled.

According to Reuters the attack took place near the town of al-Mayadeen in the vast desert province of Deir Ezzor, which is split in control between Syrian troops, 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 neighbouring Iraq from 2013, has gone underground since losing its last territory in eastern Syria in 2019.

The Assad government's official media outlets carried no immediate reports on the incident and there was no immediate claim of responsibility.

The 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. 

Despite this, 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.

In March 2019, IS lost the last territory it held in Syria to a Kurdish-led counteroffensive backed by a US-led coalition, but jihadist remnants continue to carry out deadly attacks.

Targets have included civilians and Kurdish-led fighters as well as Assad government soldiers and allied pro-Iranian fighters.

IS members in recent weeks have increased their attacks in Syria's north and northeast.

Thursday's attack was the third carried out by the jihadists this month alone.

Earlier this week, 10 Syrian and Iranian-aligned pro-government fighters were killed in an IS attack in the former jihadist stronghold of Raqqa province, the Observatory said.

Last week, the jihadists attacked a convoy of oil tankers guarded by the army in the Syrian desert, killing seven people including two civilians.

And last month, IS claimed responsibility for a rare bombing in Damascus that killed at least six people near the capital's Sayyida Zeinab mausoleum, Syria's most visited Shiite pilgrimage site.

Syrian civil war has since drawn in foreign powers, including Russian and Iranian occupation on behalf of Assad, while leading to the jihadist insurgency that birthed IS in its current form.

The conflict has killed nearly half a million people and driven half of the country's pre-war population from their homes, with many seeking refuge in neighbouring Turkey.