COLOMBO (AFP) - Sri Lanka's Tamil Tiger rebels struck back against a major government offensive with suicide attacks on merchant ships off the island's 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 crew were rescued by the navy and there were no reports of casualties among the merchant sailors. The guerrillas used three suicide boats, the official said. Sri Lanka's Tamil Tigers have a sea-going unit known as "Sea Tigers". 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 Lt-Col Ilakkiya, the deputy head of the "Black Sea Tiger" women's unit, and the other was identified as Lt-Col 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. The Defence Ministry described the attack as "another cowardly attempt by the terrorists to deny essential supplies to the civilians living in war-affected areas." The guerrillas have a history of attacking international and local merchant shipping and were blamed by the military for the killing of 24 Chinese crew members of two fishing boats in 2003. The latest attack came as the government, which pulled out of a Norwegian-brokered truce in January, maintained a large-scale ground offensive against the Tigers in the northern mainland. Government forces say they are 10 to 15km southwest of Kilinochchi, the administrative capital of the LTTE, but breached the town's major defences over the weekend. Monsoon rains and intense Tiger resistance had slowed the ground offensive, according to military sources. The Defence Ministry also said scores of its troops were killed or injured in fighting with the LTTE over the weekend. Tens of thousands of people have died on both sides since 1972, when the LTTE launched its campaign to carve out an independent state in the Sinhalese-majority island of 20 million people.