Several bombs fell south of the town of Kilinochchi, Tamil Tiger officials said. There was no word on casualties from the Sri Lankan raids.

"If they are attacking Kilinochchi, they are showing they are ready for war," S Puleedevan, head of the Tamil Tiger peace secretariat, told Reuters.

A military source said air force Kfir and MiG jets were aiming for a Tiger airstrip south of Kilinochchi from which they operate their fledgling air force - around three light aircraft smuggled into the country in pieces.

Earlier air and artillery strikes were carried out on the Tamil Tiger controlled northeast coast after two mines destroyed a bus packed with commuters and schoolchildren.

"The airstrikes were carried out as a deterrent," Prasad Samarasinghe, a military spokesman said. "We have taken known LTTE (the Liberation Tigers of Tamil Eelam) targets in Sampur and Mullaitivu."

About 500 people have been
killed in attacks since  early April

The government said rebels detonated two mines which fired ball bearings into the bus heading for Kebitigollewa, in an ethnic Sinhalese district of North-Central province close to territory held by the Tamil Tigers. Officials said 13 children were killed in the blast.

The separatists denied involvement in the attack. "Directly targeting civilians, as the Kebitigollewa  claymore attack has, cannot be justified under any circumstances," a statement said.

The bus attack was the worst single act of violence since the government and guerrillas signed a ceasefire in 2002, and renewed fears of a return to war.

Mahinda Rajapakse, the president of Sri Lanka, said he remained committed to peace as he visited victims of the attack in the government-held town of Anuradhapura.

At the hospital in Anuradhapura, people mourned the loss of entire families in the blast. "The bus was blown over," 37-year-old survivor Chintha Irangani said. She was taking her three children to a clinic on the bus and all of them died.

"This is not a declaration of war but an act of terrorism. As far as the peace process and the ceasefire agreement are concerned, they are still on, but we have to re-look at it seriously in a  proper context," Keheliya Rambukwella, a government minister said.

About 500 people have been killed since early April as talks between the government and Tigers collapsed and many now fear a slide back into the island's two decade civil war.

Tamil Tigers only returned to Sri Lanka on Wednesday after planned talks in Oslo with government officials failed to take place. The rebels refused to sit at the table with a Sri Lankan  delegation for what had been billed by peace broker Norway as two days of talks to discuss ways of ensuring the safety of  Scandinavians monitoring the truce.