Colombo  - A United Nations food convoy consisting of 50 trucks heading to rebel-held areas in northern Sri Lanka, where tens of thousands of civilians have been displaced, was forced to turn back Thursday due to firing by the rebels, the military claimed.

A military statement said that the trucks turned back at Puliyankulam, 280 kilometres north of the capital, and returned to Vavuniya, the town from where they started their journey.

The military blamed rebels for launching an "indiscriminate artillery and heavy mortar attack."

There was no independent confirmation about who was responsible for the attack.

Some 250,000 civilians are displaced in the rebel-held areas as government troops continue their attempts to recapture the areas. Most of the civilians are living in temporary shelters or open ground.

The rebels in a statement released on Wednesday claimed that the military had blown up two important bridges on which the food convoys were expected to travel, a claim denied by the military.

The government has called on civilians to leave the rebel-held areas and come into military controlled areas, but so far the response has been poor, with less than 100 families coming out.

Some of the civilians who have come out of the rebel-held areas have said that the food situation was deteriorating and one of the biggest worries is that they do not have proper shelter as the monsoon rains are expected later this month.

Government troops are reported to be outside the rebel stronghold of Kilinochchi, 370 kilometres north of the capital, and most civilians have been fleeing to the adjoining Mullaitivu district, most of which has a thick forest cover and a limited number of buildings to accommodate the displaced persons. (dpa)