Fifteen soldiers died Sunday in a terrorist attack on a pro-regime army bus in the central Syrian desert, a war monitor said, as state media reported a "terrorist attack."

Despite the fall of Daesh's "caliphate" in 2019, the group continues to launch deadly attacks from hideouts in the Syrian desert, which extends from the outskirts of the capital Damascus to the Iraqi border.

Daesh cells "attacked a military bus" in the Palmyra desert, "killing 15 soldiers and wounding 18 others," the British-based Syrian Observatory for Human Rights said.

State news agency SANA had reported 13 dead "including officers" and 18 wounded in a "terrorist attack" on a military bus on Sunday afternoon.

The Observatory, which relies on a network of sources across the country, said the death toll could rise as most of the soldiers were "seriously wounded."

Daesh did not immediately claim responsibility for the attack.

Sunday's violence came after three regime soldiers died Friday east of Palmyra when the vehicle they were traveling in came under attack, the Observatory added.

So far this year 61 pro-regime soldiers and Iran-affiliated militiamen had been killed in Daesh attacks in the desert of Syria, it said.

About half a million people have been killed and millions have been displaced since the Syrian conflict erupted in 2011, after nationwide protests against the government were met with a brutal crackdown.

It escalated into a devastating war that drew in regional and international powers.

Daesh leader Abu Ibrahim al-Hashimi al-Qurashi blew himself up in early February during a raid by U.S. forces on his house in Syria's northwest region of Idlib, Syria's last major opposition bastion.

Qurashi had taken over with Daesh weakened by years of assaults by U.S.-backed local forces and the loss of its self-proclaimed "caliphate" in Syria and northern Iraq.

The majestic ancient city of Palmyra, a World Heritage site, became the scene of public executions, where Daesh also blew up ancient monuments and looted other treasures.

Earlier in January, nine Assad regime soldiers and allied fighters were killed in an attack on a military convoy in Syria's east, while in November last year, the Observatory said another eastern Syria attack left a general and four soldiers dead.

Two bombs planted on an army bus in central Damascus killed 14 people in October last year, SANA had reported.

That was the deadliest attack in the capital since a bombing claimed by Daesh targeted the Justice Palace in March 2017, killing at least 30 people.