COLOMBO (AFP) - - The United Nations announced Tuesday it was withdrawing its aid workers from Sri Lanka's embattled north ahead of a major military onslaught, as rebels denied one of its aircraft was shot down.

Only a few aid agencies, including the UN outfits, have still been operating inside the Tamil Tiger-held Wanni district, where government forces are currently pushing to dismantle the guerrillas' mini state.

"A precise timetable for the complete withdrawal of all staff is yet to be determined, but relocations will begin this week," the UN said in a statement.

"We asked them to leave the Wanni district immediately with all their resources," the minister said. "They wanted a week or two to comply."

Colombo wants to avoid troops being accused of killing aid workers in a repeat of the August 2006 massacre of 17 local employees of the French aid agency Action Against Hunger in the east of the island.

However, the International Committee of the Red Cross (ICRC) said that it had no immediate plans to pull out.

Earlier Tuesday, the Sri Lankan government said it had for the first time shot down a Tamil Tiger aircraft after the rebels launched a major land and air strike that left 25 people dead at a key military base.

Defence officials said 12 security personnel, one policeman and a civilian were killed in the attack, while 11 guerrillas were killed by troops defending the camp.

But the rebels denied the claims, saying in a statement that "the aircraft that participated in the sortie safely returned home."

The Tigers said they lost 10 suicide bombers in the coordinated ground and air attack against the Vavuniya military complex, 260 kilometres (160 miles) north of Colombo.

Troops repelled the assault, the ministry said, adding that jet fighters had then attacked and destroyed the low-flying rebel plane.

If confirmed, it would be the first successful government strike against Tiger aircraft, which began operating in April last year.

The ministry said the Tigers used two planes to bomb the base while rebels on the ground tried to break in and attack security forces inside.

"The (rebel) aircraft has been shot down in the Mullaittivu skies while running away following a failed mission," the defence ministry said, referring to the district where Tigers operate air fields.

Squadron Leader Sanjaya Adhikari, who coordinated the government air strike, told reporters that fighter pilots had confirmed hitting the Tiger plane but that no video footage was available.

Two weeks ago the Tigers, who operate about five Czech-built Zlin-143 aircraft smuggled into Sri Lanka in parts, carried out an air strike against the northeastern port town of Trincomalee.

At least four people were killed and another 10 wounded in that attack targeting the navy.

In October last year, the Tigers launched an land-and-air attack against a key government air base and damaged a large fleet of aircraft, including spy planes.

The latest Tiger air strike came as government forces were braced for bloody battles as the military tries to capture the Tigers' territory in the north after ejecting them from the east last July.

Tens of thousands of people have died since the Tigers launched a campaign in 1972 to carve out a homeland for minority Tamils in the majority Sinhalese island's north and east.

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