Government troops pushed into rebel-held territory in northern Vavuniya district on Wednesday and destroyed the bunkers, killing eight rebels, a defense official said.
Twenty-four rebels were wounded in the battle, the official said on condition of anonymity, citing government policy.
Elsewhere in Vavuniya soldiers fired mortars at rebel locations on Wednesday, killing five guerrillas, he said.
A separate clash and an anti-personnel mine blast in the same region killed a rebel and a soldier, and wounded five other soldiers, the official said.
Also on Wednesday, soldiers attacked a group of rebels who tried to infiltrate the defense line in Muhamalai, north of Vavuniya, killing one of them, he said.
Rebel spokesman Rash Ilanthirayan could not immediately be reached for comment.
Violence has escalated in past months along the front lines surrounding the rebels' de facto state in northern Sri Lanka .
The government announced four months ago that it had driven out the rebels from Eastern Province and vowed to dismantle the rebels' ministate.
Tamil Tigers have been fighting for more than two decades for an independent homeland for the island's ethnic minority Tamils in the north and east, following a history of discrimination under governments controlled by the majority Sinhalese.
A Norwegian-brokered cease-fire in 2002 brought relative calm to the country, but a new wave of violence arose in December 2005 and has killed more than 5,000 people.
More than 70,000 people have been killed since the insurgency began in 1983.