Heavy fighting in northern Sri Lanka Thursday forced a United Nations food convoy to turn back.

The U.N. sent the 50-truck convoy loaded with food for the more than 220,000 people who have fled their homes to escape fighting between government forces and Tamil separatist rebels.

The Sri Lankan military accused the rebels of shelling the convoy, but a U.N. spokesman said only that there was heavy fighting nearby, forcing the trucks to turn around.

It was only the second U.N. aid convoy to the war-torn north since last month, when the government ordered most aid agencies out of the region.

The number of refugees in the north has surged in recent months as Sri Lanka's military has stepped up attacks against the Tamil Tiger rebels.

Separately, the Sri Lankan military says its warplanes today bombed two suspected rebel positions deep in the northern jungle.

Government accounts cannot be independently verified because the government controls media access to the war-torn northern region.

Also today, the Indian government reiterated its "grave concern" over the violence between troops and rebels.  India's foreign minister, Pranab Mukherjee issued a statement, urging the rights of civilians be respected and that food and other essential supplies be allowed to reach them.  The minister also called on the Sri Lankan government to find a peacefully negotiated political settlement with the rebels.

Wednesday, Indian Prime Minister Manmohan Singh issued a similar statement, after his allies, Indian Tamil lawmakers, threatened to quit India's parliament unless the Sri Lankan military offensive is stopped. 

Tamil rebels have been fighting since 1983 to establish an independent homeland for Sri Lanka's minority Tamil community. More than 70,000 people have been killed in the violence.

Some information for this report was provided by AFP and Reuters.