Sri Lanka's armed forces claimed today to have shot down a Tamil Tiger aircraft for the first time in 25 years of civil war after the rebels launched a land and air strike on a key military base near the frontline.

The United Nations announced later that it would withdraw its staff from rebel-held territory following a government decision to ban foreign aid workers from the region as the army closes in on the Tigers' northern stronghold.

The two developments highlight the dramatic escalation of fighting in Sri Lanka since the army intensified its offensive in the north three weeks ago, vowing to crush the rebels by the end of the year.

The Tigers attacked the base in Vavuniya, just south of the frontline and 155 miles northeast of Colombo, the capital, with artillery and ground troops at 2.50am this morning, before dropping bombs from two aircraft, according to military officials.

The Tigers, who have been fighting for an independent Tamil homeland since 1983, are the only UN-listed terrorist organisation known to have an air force, and have carried out seven air raids since unveiling it last year.

Sri Lankan officials said that the planes missed their target – a radar station – and that one was destroyed by a missile over Mullaittivu, another rebel stronghold, as they tried to return to their secret jungle base.

"The aircraft has been shot down in the Mullaittivu skies while running away following a failed mission," the Defence Ministry said in a statement.

Squadron Leader Sanjaya Adhikari, who coordinated the air force's counter-measures, told reporters that fighter pilots had confirmed hitting the Tiger plane.

But he said there was no video footage and, with journalists banned from the conflict zone, it was impossible to verify his claim independently. The Government and the Tigers routinely exaggerate losses inflicted on each other, while playing down their own.

With most communication lines cut to the north, rebel spokesmen were not available for comment.

A pro-rebel website confirmed the raid, but said there "was no indication of any aircraft coming down, according to civilian sources". It also said that three aircraft were involved.

If the loss of the aircraft is confirmed, it would be a major boost for the Sri Lankan military, which has struggled to improve its air defences since the Tigers shocked the world with an air raid on Colombo's airport last year.

It would also be a serious blow to the Tigers, who the military believes have three Czech-made Zlin-143 light aircraft, propeller-driven two-seaters, which were smuggled into the country in pieces and reassembled.

However, the scale of the attack on Vavuniya, headquarters of police and troops involved in the conflict, illustrates the rebels' determination to defend their northern territory and suggests there will be further bloodshed in the weeks ahead.

The Defence Ministry said that ten rebels, ten soldiers and one policeman were killed, mostly by artillery. Fifteen soldiers, five air force personnel and eight police were wounded, and one civilian was also killed, it said.

On Monday, the government ordered all aid workers out of the conflict zone, saying that it could not guarantee their safety as it presses ahead with its plan to eliminate the Tigers by January.

The UN said yesterday it had not set a timetable for the full withdrawal of its staff but would begin relocations this week. It vowed to continue working to address the needs of the civilian population in the region, where it estimates that 160,000 people have been displaced by the fighting.

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