By Simon Gardner

COLOMBO, March 26 (Reuters) - A newly unveiled Tamil Tiger "air force" of between two and five light planes is no match on paper for Sri Lanka's dozen fighter jets, but they pose a proven threat the military would be ill-advised to underestimate, experts say.

The Tigers smuggled small, propeller-driven planes into the island in pieces aboard merchant ships they owned after a 2002 ceasefire and reassembled them, analysts said. The truce has now broken down into renewed civil war.

Monday's pre-dawn air strike on the island's biggest air force base, close to Colombo's international airport -- the first such rebel attack -- may sound like something out of a Biggles schoolboy novel, but it should be cause for worry.

"This air attack appears to have taken the air force by complete surprise, and this is confirmed by the delayed response, by which time the attackers have been able to return to base," said Iqbal Athas of Jane's Defence Weekly.

The rebel bombs hit a barracks at the air base, 23 miles (37 km) north of the capital, killing three airmen and wounding 16. But the attack failed to destroy Air Force fighter jets, which were the target.

"It is a significant threat for a number of reasons. What they did, although they may have failed to achieve their target, is to demonstrate that they have such a capability," Athas said.

"The larger offshore patrol vessels of the navy can become vulnerable, troop transport ships can become vulnerable and so can armed groups leading an offensive on the ground."

The attack has also exposed the vulnerability of Sri Lanka's radar air defences. Experts say Sri Lanka has no access to costly 24-hour real-time satellite footage of its skies.

At least one of the Tiger aircraft is believed to be a Czech-made Z-143 two-seater training light aircraft.

Pro-rebel Web site www.tamilnet.com posted photographs of shadowy rebel leader Velupillai Prabhakaran, who has lived in hiding for much of the two-decade conflict, standing with Tiger airmen in characteristic Tiger stripes.

BIGGLES OR BATTLE STATIONS?

They also show two Tiger rebels sitting in the cockpit of a single-propeller two-seater aircraft painted in army camouflage colours, as well as the underside of an aircraft with four bombs attached to it.

"They have been caught with their pants down. It's like something out of Biggles," said one foreign security expert on condition of anonymity, referring to novels about a fictional British World War One fighter pilot.

The military was still trying to work out how the Tigers had been able to fly one or more small planes to the capital, drop bombs and make it safely home.

"That is what the Air Force is still trying to find out, where they have come from," said military spokesman Brigadier Prasad Samarasinghe. "You can come along the beach side or over water without being detected."

Samarasinghe laughed at the Tigers' claim of a new Tamil Eelam Air Force, named after the independent state they are trying to carve out in the island's north and east.

"There won't be any Tamil Eelam. Small aircraft coming and dropping one or two bombs, it's not an air force."

The Tigers, who have suffered a series of major defeats on the battlefield and at sea in recent months but retain their guerrilla warfare capability, have warned of a bloodbath across the island and have threatened more air attacks.

Observers see Monday's air raid as tit-for-tat for repeated Air Force strikes on territory the rebels control as the military pushes on with a declared plan to wipe out the Tigers militarily within 2-3 years.

"It is not only pre-emptive, it is a measure to protect Tamil civilians from the genocidal aerial bombardments by Sri Lankan armed forces," Tiger military spokesman Rasiah Ilanthiraiyan said by telephone from the rebels' northern stronghold of Kilinochchi after the sortie.

"More attacks of the same nature will follow."

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