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.