At least 26 Syrian army soldiers were killed and more than 10 were wounded when ISIS fighters ambushed their bus overnight in eastern Syria, a war monitor said.
The attack, the deadliest of its type this year, occurred near Al Mayadeen in the desert province of Deir Ezzor, the UK-based Syrian Observatory for Human Rights said.
The war monitor said the death toll would probably rise, with the status of dozens of Syrian army soldiers unknown.
The official Syrian news agency Sana said “a number of military personnel were killed and others injured” in a “terrorist attack”, citing an army source.
Syrian government forces and allied pro-Iranian armed groups deployed in Deir Ezzor were on high alert on Friday morning, the Observatory reported.
The area was once controlled by ISIS during the period that the group ran parts of eastern Syria and northern Iraq.
However, since losing the territory they held – in Iraq in 2017 and Syria in 2019 – the extremist group has returned to being an insurgency with underground sleeper cells.
Control in Deir Ezzor is split between Syrian troops, backed by Iran, Russia and Lebanon's Hezbollah, and Kurdish-led fighters, supported by the US and other western allies.
ISIS in recent weeks has increased its attacks in Syria's north and north-east.
Thursday's attack was the third by ISIS this month, amid an apparent resurgence by the extremist group.
Also this week, 10 Syrian soldiers and pro-government fighters were killed in an ISIS attack in the former stronghold of Raqa province, the Observatory said.
Last week, ISIS attacked a convoy of oil tankers guarded by the army in the Syrian desert. That killed seven people, including two civilians.
And last month, ISIS 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.