COLOMBO, SRI LANKA - TAMIL Tiger rebels said Sri Lankan troops on Tuesday ambushed a school bus and killed 18 people, including 11 children, as the military reported killing 67 militants in two days of heavy fighting.

The Liberation Tigers of Tamil Eelam (Ltte) said a 'deep penetrating unit' of the military carried out the bomb attack against the school bus in the district of Mannar where heavy fighting has been raging for weeks.

'All the passengers in the bus were either school children or teachers returning from a sporting event,' the Ltte said in a statement. 'Among those killed were two teachers and the school principal. One of the teachers killed is a Catholic nun.'

Earlier in the day, the Sri Lankan defence ministry said its soldiers had advanced into territory held by Tamil Tiger rebels in the Mannar district.

The defence ministry said its troops smashed 16 guerrilla bunkers in Mannar on Tuesday and killed at least 22 rebels.

Security forces killed 45 rebels along the northern frontlines on Monday, the ministry said, putting its own losses at two soldiers dead and 10 wounded.

There was no immediate comment from the Ltte about the fighting and the casualty claims, but the pro-rebel Tamilnet.com website said the guerrillas were using booby-trapped devices and snipers against attacking troops.

Since the beginning of January, the Sri Lankan defence ministry says its forces have killed 815 rebels for the loss of just 30 soldiers.

Both sides are known to give wildly varying casualty figures which cannot be independently verified. The government bars reporters from visiting frontline areas and rebel-held territory.

Sri Lanka pulled out of a Norwegian-brokered truce with the rebels this month, underscoring its belief that it has the upper hand in the drawn-out conflict.

Tens of thousands of people have died since the Ltte launched its campaign to carve out an independent homeland for minority Tamils in the majority Sinhalese nation in 1972. - AFP