REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE CONTENT OF THIS VIDEO, WHICH HAS BEEN OBTAINED FROM A SOCIAL MEDIA WEBSITE.

An aid convoy comes under fire in the besieged Syrian city of Homs.

It happened late Saturday when mortar fire and gunshots hit the convoy of at least nine trucks.

They were from the United Nations and the Syrian Arab Red Crescent, on their way to a rebel held district to deliver food and medicine to civilians trapped in the fighting for months.

Reuters can't independently verify this video, which was posted online and purports to be the aftermath of the attack.

It's unclear who launched the attack.

Both sides blame the other.

Syrian TV said at least four aid workers were wounded.

The attack violates a three-day ceasefire designed to get much-needed food and supplies inside Homs.

It also threatens to unravel a humanitarian deal that included the evacuation of civilians.

The deal had been the first concrete