COLOMBO (AFP) - Troops and Tamil Tiger rebels were locked in intense fighting in northern Sri Lanka Sunday after govt forces smashed through the "last major defences" of the guerrillas, the military said. The rebel Liberation Tigers of Tamil Eelam (LTTE) launched "poisonous gas attacks" to blunt the military offensive aimed at capturing the rebels' political capital of Kilinochchi, the defence ministry said in a statement. Soldiers stepped up their offensive on Saturday and captured a two-kilometre (1.25-mile) stretch of the defence line set up by the Tigers near Kilinochchi, the ministry said. It said the fighting was at Akkarayankulam, about 13 kilometres southwest of Kilinochchi. Three weeks ago, the military had said it was within striking distance of Kilinochchi."Army's 57 division troops yesterday pierced the LTTE-built earth bund in Akkarayankulam, the terrorists' last major defence south of Kilinochchi," the defence ministry said in a statement. It said the Tigers "launched poisonous gas attacks" on the troops. Military sources said that the Tigers had used a type of tear gas commonly used during riot control. It gave no details of casualties for both sides, but added: "Several soldiers made their ultimate sacrifice for the nation while many others suffered injuries during this battle." In separate statement, the ministry said soldiers along two other fronts in the north of the island had also stepped up attacks against Tamil Tiger positions over the weekend. Soldiers had encircled a sea base of the Tamil Tigers along the northwest coast while troops on the other side of the island in northeastern Mullaitivu had also stepped up attacks, the ministry said. "Troops also suffered casualties during those confrontations and were being evacuated for treatment," the statement said. Government soldiers fell victim to booby-trapped devices, anti-personnel mines and mortar fire, the ministry said.