(Updates with background, details throughout)

COLOMBO, March 29 (Reuters) - Sri Lanka's navy said on Thursday it sank three Tamil Tiger boats off the island's northeast coast, killing between 15 and 18 rebel fighters.

The attack late on Wednesday off the coast of the rebel-held northeastern district of Mullaithivu is the latest in a rash of land and sea battles after the resumption of a two-decade civil war that has killed around 68,000 people since 1983.

"A naval patrol has come across about 10 Tiger boats and our boats launched an attack, and we were able to destroy three rebel craft," said Commander Athula Senaratne, a navy spokesman. "During the attack we believe 15-18 rebel cadres were killed."

The Tigers were not immediately available for comment. The attack came after the military said troops had driven the rebels from a stronghold in the island's restive east.

The Tigers, who are seeking to carve out an independent state for minority Tamils in the island's northeast, have lost an estimated 600 square kilometres of terrain in the face of military offensives in the east in recent months.

President Mahinda Rajapakse's ethnic Sinhalese-majority government is pushing on with a drive to destroy the Tigers militarily, despite international community calls to respect a tattered 2002 ceasefire.

The Tigers have warned of a bloodbath and analysts expect a war that has killed around 4,000 people in the past 15 months alone to escalate.

A Tiger suicide bomber tried to blow up an army camp in Sri Lanka on Tuesday, killing nine people. The day before, rebels carried out their first air strike in a daring attack on an air base near the capital's international airport.

The government has yet to explain how the Tigers managed to fly a light aircraft over the area undetected, drop bombs and fly back to their northern stronghold without being shot down.

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