COLOMBO (AFP) - - UN chief Ban Ki-moon expressed international concern for tens of thousands civilians trapped in Sri Lanka's north Wednesday as government forces prepared for a major showdown with Tamil separatists.

Aerial strikes shook Tiger positions a day after the guerrillas staged an air and ground assault that left 25 people dead at a military complex just outside the rebels' mini state, officials said.

Fighter planes pounded what the Sri Lankan military called the Tiger intelligence command and control centre, deep inside the rebel-held Kilinochchi region, the defence ministry said in a statement.

"The secretary general expresses his deep concern over the increased hostilities in northern Sri Lanka, and the grave humanitarian consequences for civilians," his press office said in a statement released in New York.

The statement came as Colombo told all local and international relief workers, including UN and Red Cross staff, to vacate the region within a week, saying the order was for their own safety.

The London-based rights group Amnesty International said the government lacked the capacity to provide basic essentials and safety for those who have had to flee deeper into rebel-held territory.

"Without independent monitors in the region, there will be a complete void of information," said Sam Zarifi, Amnesty International's Asia Pacific Director.

The defence ministry said its planes had hit key Tiger targets.

"Taking on offensive raids into the LTTE (Liberation Tigers of Tamil Eelam) backyard, Sri Lanka air force fighter jets made precision air sorties at the LTTE's main intelligence command and control centre in Kilinochchi on Wednesday," the ministry said in a statement.

It said 23 Tiger rebels and two soldiers were killed in fresh ground attacks on Tuesday in addition to the fighting that took place at the main military complex of Vavuniya, 260 kilometres (160 miles) north of Colombo.

The latest fighting raised the number of rebels killed since January to 6,466, according to government figures.

The military says 631 soldiers have died over the same period in a conflict that has claimed tens of thousands of lives since 1972.

Sri Lankan planes carried out at least four other bombing sorties after the Tiger rebels on Tuesday staged their pre-dawn attack on the Vavuniya complex.

The government said it had for the first time shot down a Tamil Tiger aircraft following the Vavuniya strike, a claim denied by the rebels.

The Tigers said their mission to destroy air defence radar of the security forces was a success, while the defence ministry described the rebel offensive as a failure.

The ministry said the Tigers fired about 70 rounds of artillery into the base while a group of suicide bombers tried to infiltrate it.

"The terrorists' barrages of 130mm artillery could not prevent the army counter offensive and the whole mission became a flash in a pan as troops hunted down all the 'Black Tigers' (suicide bombers)," the ministry said.

The Tigers are fighting to carve out a homeland for minority Tamils in the majority Sinhalese island's north and east.

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