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Nato has launched the second phase of its air strikes campaign against Yugoslavia, directly targeting its armed forces for the first time.

"We are now just beginning to transition to that phase two and address more targets in Kosovo," Air Commodore David Wilby told a news conference at Nato's Brussels headquarters. "We have started."

As the alliance warned of an "imminent humanitarian catastrophe" with more than 500,000 people displaced by a Serb offensive in Kosovo, it said it had stepped up its attacks with a "broader range of air operations".

Four Serbian tanks were blown up near Malisevo in central Kosovo, according to Kosovo Albanian sources.

There were also attacks reported on targets in Pristina, and in other parts of Serbia.

Nato said on Saturday that it would attack targets including Yugoslav combat troops in and around Kosovo. Tanks, artillery, transporters and mobile command centres would be hit under phase two of Operation Allied Force.

Jonathan Marcus: The alliance is switching the emphasis of its attacks

Air Commodore Wilby said Nato warplanes had made 253 sorties over Yugoslavia on Saturday - the fourth day of the Allied campaign. The planes had attacked 11 targets around the capital Belgrade, and six others scattered across the country.

He said the warplanes had hit air defence systems and had also targeted military and special police (MUP) command facilities.

Warning of humanitarian catastrophe

Alongside the Yugoslav army, the MUP is said to be sweeping through Kosovo driving much of the local Albanian population from their homes.

In recent days some 20,000 people have been driven from towns in northern and central Kosovo and are trying to cross into Albania.

Nato spokesman Jamie Shea said that the Albanian Government had accepted up to 10,000 refugees in just the last few hours.

All 19 Nato member governments had quickly given their assent to expanded operations, Mr Shea said, and were resolved to see the campaign through.

More aircraft

The UK is among those who have pledged extra support to bolster the air strikes. It has sent 13 extra aircraft in a bid to halt President Slobodan Milosevic's "campaign of genocide".

UK Defence Secretary George Robertson promised four more Harrier jump jets and eight Tornado GR1 bombers. An additional Tristar tanker is also being sent to join the air-to-air refuelling fleet.

Nato troops in Macedonia: Holding their ground

He said reports from refugees and aid agencies on the ground "are enough to convince us that we are confronting a regime which is intent on genocide".

Air Commodore Wilby said he was hoping to be able to announce details of further increased military contributions from Allied nations on Monday.

BBC Defence Correspondent Mark Laity reports that sources suggest many of the Serbs' air defences - while still a threat - have been badly hit, allowing Nato more scope to focus its attacks on Serb forces in Kosovo.

Nato loses warplane

It remained unclear whether it was those air defences which had shot down a US Stealth fighter plane on Saturday or whether it had crashed due to mechanical failure.

John Simpson reports: "Serbian authorities finally found a use for the foreign media"

Air Commodore Wilby said the F-117A - the first Nato plane lost in this campaign - went down at 1945 GMT on Saturday.

He said Nato forces had mounted a "very successful" combined rescue operation and that the pilot of the plane was in good health at Aviano air base in Italy.

Several blasts rocked Belgrade on Sunday and air raid sirens sounded from the early morning. Yugoslav state news agency Tanjug said missiles landed near two airports in the suburbs. There were also reports of attacks on military targets in Cacak, Prizren and Pristina.

The all clear sounded six hours later, but fresh air raid alerts were reported around 1450 GMT.

Yugoslavia upbeat

President Milosevic remained unbowed in the face of both military and verbal onslaughts on his regime.

He held a meeting with the top state and military leaders in Belgrade on Sunday and they gave an upbeat assessment of the situation in the country.

They pronounced Yugoslavia fit to continue resisting Nato air attacks, according to Tanjug.

They found coordination between civilian and military bodies "excellent", and this had "strengthened all elements significant to the defence of the country", the news agency said.