Islamic State group fighters on Thursday killed 28 Syrian soldiers and affiliated pro-government forces in two attacks on government-held areas of the war-torn country, a war monitor said.

In one attack, "22 regular army soldiers and fighters from pro-government forces were killed when IS jihadists opened fire on a military bus in eastern Homs province", said the Syrian Observatory for Human rights.

Most of those killed were members of the Quds Brigade, a group comprising Palestinian fighters that has received support from Damascus ally Moscow in recent years, according to the Britain-based monitor.

Separately, "six Syrian soldiers died in another IS attack against a base near Albu Kamal," in eastern Syria, the monitor reported.

State media has yet to report on the attacks.

IS overran large swathes of Syria and Iraq in 2014, proclaiming a so-called caliphate and launching a reign of terror.

It was defeated territorially in Syria in 2019, but its remnants continue to carry out deadly attacks, particularly against pro-government forces and Kurdish-led fighters in the vast Badia desert.

In late March, IS militants killed eight Syria soldiers after an ambush, the monitor had said, also reporting 14 troops killed by the jihadists in prior days.

Syria's war has claimed the lives of more than half a million people and displaced millions more since it erupted in March 2011 with Damascus's brutal repression of anti-government protests.