The so-called Islamic State group (IS) carried out an attack that resulted in the deaths of at least 33 soldiers who supported Syrian President Bashar al-Assad.
Two army trucks were ambushed overnight on Thursday in the eastern region of Deir el-Zour.
It is among the bloodiest acts of extremism committed this year, according to the Syrian Observatory for Human Rights (SOHR), a war monitoring organisation with headquarters in Britain.
There have been 33 deaths in the last two days, up from 23, but there are worries the figure will go up.
There were at least 10 additional victims of injuries, some of whom are still in critical condition.
35 people were killed, according to another activist group that reports on events in the east, and all of the victims were soldiers from the 17th Division of the Syrian army.
Statistics from SOHR show that since the beginning of 2023, 412 people have died during military operations in the Syrian desert.
The ambush was claimed by IS in a message posted on Telegram on Friday night.
Let the entire world know that we show our commitment to our leaders via deeds rather than words and that our jihad will continue till the end of the world.
This occurs a week after the apparent leader of the extreme group, Abu al-Hussein al-Husseini al-Qurayshi, who had been in charge since November, passed away in Syria.
Since the organization’s founder, Abu Bakr al-Baghdadi, was murdered by US troops in the nation’s northwest in 2019, he was the fourth leader to be assassinated.