At least 541 insurgents and 44 soldiers have been killed in battles in Mannar and Vavuniya districts, military spokesman Brig. Prasad Samarasinghe said.

He said the aim of the operation is to destroy Tamil Tigers mortar positions and to free thousands of civilians held as "human shields" in the rebel-held territory.

Rebel officials could not be immediately reached for comment.

The two districts lie at the edge of a vast rebel-held area where the Tigers claim to run a de-facto state with their own police and judiciary.

Also the military said on Sunday that soldiers pre-empted an attempt by Tamil Tiger rebels to infiltrate a defence line elsewhere in the north, killing three guerrillas.

Troops manning the defence line in Muhamalai on the northern Jaffna peninsula, 185 miles north of the capital Colombo, killed the rebels Saturday evening, an official from the Defence Ministry's information centre said on customary condition of anonymity in line with policy.

Muhamalai also lies between the government-held peninsula and the rebel territory.

The clashes are part of a worsening separatist conflict on the island, with 5,000 people killed in the past 18 months shattering a Norway-brokered cease-fire signed five years ago with strong international backing.

Both sides have said the cease-fire has become irrelevant but neither have withdrawn from it.

The Liberation Tigers of Tamil Eelam rebels have fought the government since 1983 to carve out an independent homeland for ethnic minority Tamils who have suffered discrimination by the majority Sinhalese dominated state.

About 70,000 people have been killed in two decades of fighting.