Stepping up its offensive against the Tamil Tigers, Sri Lankan navy on Wednesday attacked a suspected Liberation Tigers of Tamil Eelam gun running vessel and engaged a flotilla of the rebels off the island's northeast, killing 15 guerrillas.

The navy detected the boat off Dondra and despite repeated warnings to change course and arrive at a port for inspection, the vessel tried to get away, a navy spokesman said.

He said the vessel began firing at the navy when it fired warning shots.

"There were some explosions aboard the suspicious vessel and we think they were transporting weapons and explosives," the spokesman said.

The attack came after the navy engaged a flotilla of Tamil Tiger boats off the island's northeast and sank one boat, killing 15 rebels.

In a confrontation on land in the island's north-western district of Mannar, three rebels were shot dead Wednesday morning, the military said.

The pre-dawn confrontations with the LTTE off Pulmoddai wounded at least two sailors on the navy side, the spokesman said, adding that rebel casualties were not immediately known.

The clash came one day after the Tigers shelled two military helicopters carrying ambassadors from France, Germany, Italy, Japan, the United States, European Union and the heads of UN agencies.

The Italian and US ambassadors and the UN resident coordinator were lightly injured.