Syria's five-day-old ceasefire appears to be crumbling on Tuesday, with the Syrian government launching a barrage of shells and mortars into the already-battered opposition stronghold of Homs.

This video, posted to a social media website by activists - and which cannot be independently verified by Reuters - shows smoke rising from buildings, purportedly in the districts of Bayadiya and Khalidiya.

The violence comes after the arrival in Damascus of an advance team of United Nations monitors.

Their task will be to judge compliance with a truce that has brought only short-lived breaks in violence since President Bashar al-Assad pledged to enforce it last week.

Travis Brecher, Reuters