Nine members of the Sri Lankan military and 20 Tamil Tiger ground fighters have been killed after the rebels attacked an air force base, the military says.
The Tigers' leader and the fighters said to have taken part in the raid
Four more airmen were killed when their helicopter sent to provide back-up crashed in the area.
Two rebel planes dropped bombs on the base at Anuradhapura, north of Colombo, in what was the Tigers' first combined air and land operation.
The fighting on Sri Lanka's northern fronts has intensified recently.
The Tamil Tigers are fighting for a Tamil homeland in the north and east of the island.
The rebels said they still held parts of the base, 210km (130 miles) north of the capital, hours after the attack. The military say they have regained complete control of the base.
Tamil Tiger military spokesman Rasiah Ilanthirayan said 21 men and women infiltrated the base at Anuradhapura, an ancient capital and tourist attraction well inside government-held territory.
The rebels said the fighters were part of the Black Tigers, a suicide squad responsible for scores of bombings and assassinations, as well as attacks on the land and at sea.
They released a photo of the 21 fighters taken with the Tigers' leader, Velupillai Prabhakaran.
While the fighting was taking place on the ground, two planes belonging to the rebels flew overhead and dropped bombs.
Defence spokesman Keheliya Rambukwalla told reporters in Colombo that eight members of the air force and one member of the army died in the attack.
He said the bodies of 20 bodies of Tamil Tiger fighters had been found.
The military insisted that the helicopter that crashed near the base went down because of a technical fault, not gunfire.
Rebel spokesman Mr Ilanthirayan said the planes involved in the attack got away, as they did after three air raids earlier this year.
The rebels are believed to have a small fleet of Czech-made two-seater propeller driven aircraft.
They are thought to have been smuggled into Sri Lanka in pieces, reassembled and modified to carry bombs.
Earlier this year, the Tigers used such a plane to bomb an air base next to the country's only international airport, near Colombo.