Damascus: An explosion that led to a building collapse in an opposition-held area of Idlib city has killed 31 people, activists said on Tuesday.

The cause of the blast that hit the city on Monday, killing 22 civilians and an estimated nine militiamen, was yet to be ascertained, UK war monitor Syrian Observatory for Human Rights was cited as saying by Efe news.

Members of the voluntary group Syria Civil Defence, better known as the White Helmets, scoured the rubble for victims.

Idlib province is almost entirely controlled by Islamist rebel groups, many of which are linked to Al Qaeda.

The region has also become a destination for thousands of civilians evacuated from other opposition-held pockets around Syria.