A deadly attack on a bus carrying Syrian soldiers is believed to have been carried out by ISIL (ISIS), which lost the last territory it held in Syria in 2019.

The Britain-based Syrian Observatory for Human Rights said on Friday that 23 Syrian soldiers were killed and 10 were wounded in the attack on a desert road near the eastern town of Mayadeen, in Deir Az Zor province that borders Iraq.

Other Syrian activist groups said 20 soldiers were killed and others were wounded.

The Britain-based observatory said the ambush was carried out by members of ISIL (ISIS), whose sleeper cells in parts of Syria still carry out deadly attacks despite their defeat in 2019.

It added that “dozens of [other] soldiers” were missing after the armed men surrounded the bus and opened fire, in what was the deadliest attack so far this year by ISIL.

There was no immediate comment from the Syrian army or government.

Attacks by ISIL sleeper cells in Syria, particularly in the vast desert zones they once controlled, have become bolder and bloodier in recent months, according to the observatory head, Rami Abdurrahman.

Abdurrahman said ISIL “has recently been escalating its deadly military attacks … aiming to cause as many deaths as possible”.

By doing so, they are trying to send “a message aimed at showing the group is still active and powerful despite the targeting of its leaders”, he told AFP.

The group named Abu Hafs al-Hashimi al-Qurashi as its new leader this month, for the first time confirming the death of its former head Abu Hussein al-Husseini al-Qurashi, whom Turkey said it had killed in April.

Its members controlled large parts of Syria and Iraq where they declared a caliphate in June 2014. Over the years they lost the territory and were defeated in Iraq in 2017 and two years later in Syria.

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

The vast desert province of Deir Az Zor is split in control between Syrian troops, backed by Iran and Russia, and Kurdish-led fighters, backed by the United States.

ISIL targets have included civilians and Kurdish-led fighters as well as government troops and allied pro-Iranian fighters.