The suicide attackers pose with rebel leader Prabhakaran before the attack
Sri Lanka's military says 12 soldiers and a policemen have been killed during a Tamil Tiger attack on a base in the northern area of Vavuniya.
The Tigers say 10 of their suicide fighters were killed in the raid.
The government says a Tiger plane was shot down, a claim which the rebel group has denied.
Meanwhile, the UN says its staff will begin leaving rebel-held territory this week, after a government order banning foreign aid workers from the area.
The Ministry of Defence says the pre-dawn land and air attack by the Tigers on an army and air force complex was "completely foiled".
It said that 10 Tamil Tigers had been killed, as well as 12 soldiers and one policeman.
Sources in the Sri Lanka air force also said they had shot down a Tamil Tiger plane in the Mullaittivu region. If confirmed, it would be the first rebel plane downed by the military. The media are denied access to the area.
The Indian High Commission in the Sri Lankan capital, Colombo, told the BBC Tamil service that two Indian radar technicians working at the air base had been injured in the attack. Its spokesman Thinkar Asthana said the technicians were doing routine maintenance and servicing work in the complex.
That has been denied by the Sri Lankan army.
In their account of the fighting, the Tigers said that 10 of their suicide attackers died and that they had destroyed a Sri Lankan air force radar station. They said at least 20 soldiers had been killed and they denied that any of their planes had been shot down.
Weapon stores, a communication tower, a communication facility and anti-aircraft guns were destroyed in this attack, the Tigers' military spokesman, Rasiah Marshall, said.
The Tigers released a photograph of the suicide attackers, taken with Tamil Tiger leader Prabhakaran, before the attack and said the suicide squad had been assisted by other fighters on land and in the airplanes.
The Tigers' rudimentary air force began operations last year with a surprise attack on an air base on the outskirts of the capital, Colombo.
The BBC's Roland Buerk in Colombo says the Tamil Tigers have a number of small Czech-built, two-seater, propeller-driven Zlin-143 aircraft, which are operated from jungle airstrips.
They are thought to have been smuggled into the island in pieces, then reassembled and modified to carry bombs, our correspondent says.
Tuesday's clash comes as government forces continue a major offensive against the rebels in northern areas of the island.
On Monday, the government ordered all aid workers out of the battle zone, saying it could not guarantee their safety.
The Tamil Tigers have been fighting for a separate state for the Tamil minority in the north and east of Sri Lanka for 25 years. More than 70,000 people have died.