This file photo frame grab from video posted online March 18, 2019, by the Aamaq News Agency, a media arm of the Islamic State group, shows an IS fighter firing his weapon during clashes with the U.S.-backed Syrian Democratic Forces (SDF) fighters, in Baghouz, Syria. AP
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, backed by Iran and Russia, and Kurdish-led fighters, backed by the United States.
Despite losing their last piece of territory in Syria in 2019, IS has maintained hideouts in the vast Syrian desert from which it has carried out ambushes and hit-and-run attacks.
IS "members targeted a military bus" in Deir Ezzor province on Thursday, said the Syrian Observatory for Human Rights.
"The number of dead has risen to 26" soldiers, it said, revising an earlier toll of 23 and calling it the extremists' deadliest attack on government forces this year.
Eleven other soldiers were wounded, with some in critical condition, while troops previously thought to be missing were on other buses that managed to reach safe areas, said the Britain-based group which relies on a network of sources inside Syria.
However, there was no immediate claim of responsibility.
The official Syrian news agency SANA said the "terrorist attack" had caused a number of military casualties, citing an unidentified army source.
Syrian government forces and their allied armed groups deployed in the area were on high alert on Friday morning, the Observatory reported.
The war monitor's Rami Abdel Rahman said IS "has recently been escalating its deadly military attacks... aiming to cause as many deaths as possible".
By doing so, the jihadists are trying to show that IS "is still active and powerful despite the targeting of its leaders", he told AFP.
Last week, IS announced the death of its leader Abu al-Hussein al-Husseini al-Qurashi, who it said was killed in clashes in northwestern Syria.
A spokesman for the group announced a new leader, known as Abu Hafs al-Hashimi al-Qurashi, in a recorded message on its channels on the Telegram messaging app.
Spate of IS attacks
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 government troops 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 soldiers and pro-government fighters were killed in an IS attack in the former jihadist stronghold of Raqa 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.
IS has had five leaders since it lost the last remnant of the once sprawling "caliphate" it proclaimed across large swathes of Syria and neighbouring Iraq in 2014.
Four of them were killed, including the group's first "caliph", Abu Bakr al-Baghdadi, who died in a US raid in October 2019.
Civil war first broke out in Syria after President Bashar al-Assad's government crushed peaceful protests in 2011. It has since drawn in foreign powers and global jihadists.
The conflict has killed more than half a million people and driven half of the country's pre-war population from their homes, with many seeking refuge in neighbouring countries.