Five Sri Lankan soldiers were killed and another six were injured in a ground battle during the first military advance on Tamil Tiger territory since a 2002 truce.
The military said it believed dozens of Tiger fighters had also been killed during the fight, but there was no immediate confirmation from the rebels.
A senior leader of the Tigers' political wing today said Sri Lanka's four-year ceasefire is now void and that the island's two-decade civil war is back on.