COLOMBO Sri Lankas Tamil Tiger rebels struck back against a major government offensive with suicide attacks on merchant ships off the islands northern coast on Wednesday, defence officials said.

The Liberation Tigers of Tamil Eelam (LTTE) rammed explosives-laden boats against the MV Ruhuna and MV Nimalawa which were supplying the besieged Jaffna peninsula in a pre-dawn attack, officials said.

The ethnic guerrillas also fought a sea battle with naval units defending the port of Kankesanthurai on the peninsula and the convoy of merchant vessels.

At least six members of the elite Black Sea Tiger suicide squad may have perished in the attack, officials said.

'One of the merchant vessels -- MV Nimalawa -- was crippled and the other was damaged,' said a defence official who declined to be named. He said a salvage operation was under way.

He said the crew was rescued by the navy and there were no reports of casualties among the merchant sailors, who were escorted by heavily armed navy troops.

The guerrillas used three suicide boats, the official said. One of them capsized and the navy managed to capture it without its occupants, who were believed to have been killed in the naval firing.

Sri Lankas Tamil Tigers have a sea-going unit known as 'Sea Tigers,' a rarity among rebel outfits in the world. The Tigers have used explosives-laden boats to sink naval and civilian craft in the past as part of their drawn out and deadly campaign for an independent state for ethnic minority Tamils.

There was no immediate comment from the Tigers, but the pro-rebel website Tamilnet.com said two Black Sea Tigers, one a woman, died in the attack.

One was Lieutenant Colonel Ilakkiya, the deputy head of the 'Black Sea Tiger' womens unit, and the other was identified as Lieutenant Colonel Kuperan.

Tamilnet quoted an unidentified Tiger official as saying that the ships were carrying military supplies as well as other cargo, but Sri Lankan officials rejected the claim and said the vessels only carried supplies for civilians.

The Jaffna peninsula, which has a population of nearly half a million, is controlled by government forces but is cut off from the rest of the island by LTTE-held territory and supplied entirely by ship or plane.