At least 52 civilians were killed in shelling in Sri Lanka's embattled northern Wanni region, where a hospital came under attack for the fourth straight day today - this time by a cluster bomb, forcing authorities to evacuate patients from one of the last functioning medical facilities in the war zone.

"We have a confirmed report that 52 civilians have been killed in a shell attack at Suranthapuram (in Wanni region) last evening," UN Spokesman Gordon Weiss said, adding it was not known who was responsible for the attack.