A UN convoy carrying food for 200,000 people trapped by fighting entered Sri Lanka's war zone on Thursday, and the air force said it bombed the headquarters of the Tamil Tiger rebels.

A UN convoy of 51 vehicles carrying 800 metric tonnes of food aid crossed the Omanthai checkpoint, the north of which is LTTE-held territory, on Thursday and was due to deliver food to four areas east of Kilinochchi where most of the refugees are.

It originally had 60 trucks, but the number was reduced after the military had found explosives and other contraband inside nine of the vehicles, the U.N. said.

The UN said it "condemned the attempt by persons unknown to disrupt the aid effort" by placing the illegal cargo on trucks provided by the government.

Sri Lanka's government and military routinely accuse aid groups of assisting the rebels, which the humanitarian community denies.

The government last month ordered nearly all aid agencies out of the war zone last, saying it could not guarantee their safety. But diplomats pressed the government to allow the food in, saying too many people were at risk.

Agencies