Nato airforces have hit Serbian armoured forces on the ground in Kosovo in the first wave of air strikes since Yugoslavia offered a ceasefire.

Briefing reporters at Nato headquarters, Air Commodore David Wilby said allied aircraft flew 439 sorties on Tuesday striking 28 fixed targets.

Mr Wilby gave photographic evidence of Nato's "first major breakthrough against armoured forces in the field."

In one attack, he said, allied planes dropped weapons on a column of seven to 12 vehicles.

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"The weather remains good and we continue to put heavy empahasis on our operations against forces in the field," said Mr Wilby.

He also detailed overnight strikes on a militia hangar facility in Belgrade and an attack on Pristina airfield in which three MiG fighters were destroyed on the ground.

Intensified campaign

Earlier, the UK Foreign Secretary, Robin Cook said the good weather had enabled Nato forces to "ramp up" their attacks on Yugoslavia.

"Over the 15 days of the air campaign some 190 attacks have been conducted. Over 80 of them have been in the last four days. That demonstrates the increasing tempo of our military campaign," said Mr Cook.

He also dismissed Serbia's offer of a unilateral ceasefire as a sham, saying if President Slobodan Milosevic wanted peace he would have to try harder.

He said the offer was the first sign that President Milosevic is "faltering".

Heavy bombing

Serbian media reported Kosovo had been under heavy attack with hits around Pristina on a fuel depot and the airport.

Hits were reported in Pristina

Serbia's RTS television station reported fires blazing at residential apartment blocks in the centre of Pristina.

Several residents were allegedly killed and wounded in the attacks.

Bombing was also reported around Belgrade, the northern town of Novi Sad, and in the south, where a chemical factory at the town of Lucani was set on fire.

Local sources say the power supplies to Lucani have been cut.

A residential district of Montenegro's capital, Podgorica, has also come under attack. There were no injuries but several buildings were damaged, according to reporters at the scene.

Ceasefire rejected

The latest strikes came as Serbia's ceasefire move was rejected by Nato and its individual member states.

The US said it was "not only completely unacceptable but absurd". The UK called it a "diplomatic ploy".

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Nato's Secretary-General Javier Solana said: "Before a ceasefire can be considered, President Milosevic must meet the demands established by the international community."

Nato officials say a ceasefire offer must be accompanied by a withdrawal of Yugoslav forces from Kosovo, a return of refugees and a halt to the repression of ethnic Albanians in the province.

The official ceasefire statement from the Serbian and Yugoslav Governments, said its ceasefire would come into force from 2000 local time (1800GMT).

Serbian TV reported that all Yugoslav army units in Kosovo were complying with the order from Belgrade.

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The Yugoslav Ambassador to the United Nations, Vadislav Jovanovic, said that the rejection of the offer is a "public admission that Nato countries are more interested in continuation of the war than in seeking peace".

"Their insistence on such a position is evidence that arrogance takes precedence over common sense," said Mr Jovanovic.

The Yugoslav statement pledged future discussions with the moderate ethnic Albanian leader, Ibrahim Rugova, on a final settlement to the crisis.

It also promised that Belgrade would work for the return of about 400,000 ethnic Albanians who have fled Kosovo.