A bomb attack on a Syrian army bus near Damascus Thursday killed at least 18 soldiers and wounded 27 others, state media said, in one of the deadliest such operations.

"A military bus in the suburbs of Damascus was hit by a terrorist bombing using an explosive device that was planted previously, which led to the deaths of 18 soldiers" the official SANA news agency said.

"27 others were wounded," the report added without elaborating.

It was not immediately clear who was behind the latest of a series of bombings targeting Syrian army buses, mostly by jihadists.

In June, a bus attack claimed by the Islamic State group killed 13 Syrian soldiers in the northern province of Raqqa.

In March, 15 soldiers were killed in an IS attack on an army bus in the central Syrian desert, according to the Britain-based Syrian Observatory for Human Rights.

In October, a bomb attack on an army bus in Damascus killed at least 13 people. It was not claimed by any group.

The Syrian conflict that erupted in 2011 has left around half a million people dead, according to the Syrian Observatory for Human Rights.

Half of Syria's pre-war population of 22 million have been forced to flee their homes.