•  Click here for live coverage
  
• Click here for map of main targets

Nato forces have been authorised to attack a broader range of targets inside Yugoslavia.

The decision comes amid growing concern over reports of atrocities by Serbian forces in Kosovo.

As many as 30,000 refugees are believed to be stranded without shelter inside the province, with thousands more crossing into neighbouring Albania and Macedonia.

Bridget Kendall: New urgency to Nato's campaign

Nato's move to the second phase of its military campaign means that commanders will be able to target tanks and other facilities.

Up to now the air attacks have been aimed at the control and supply centres that make up Yugoslavia's air defence systems.

Nato spokesman Jamie Shea explains the new strategy

Nato spokesman Jamie Shea said the intention was to target tanks and artillery that were being used against civilians in Kosovo.

A Nato statement said the decision was approved by all the allied countries involved.

Riskier strategy

BBC Defence Correspondent Mark Laity says the decision involves risk, as Nato aircraft will have to fly lower to hit their targets.

The Nato statement insisted it was not at war with Yugoslavia, and said President Slobodan Milosevic could halt the air raids by agreeing to international demands on Kosovo.

UK Prime Minister Tony Blair has pledged to increase Britain's air power contribution to the Nato air strikes.

Mr Blair made this promise during a conversation with United States President Bill Clinton, the prime minister's office said.

Fourth day of attacks

The decision to step up the campaign against Yugoslavia comes at the end of the fourth day of air strikes against Yugoslav targets.

A senior Nato official in Washington said strikes began on Saturday with a Tomahawk cruise missile launched onto the vicinity of the capital, Belgrade.

US officials later reported that American warships in the Adriatic had launched two Tomahawk missiles in daylight.

Nato forces were continuing to prepare their aircraft on Saturday

Air-raid sirens sounded in Belgrade and in Pristina and large explosions were reported in the Yugoslav capital.

Witnesses reported scores of people rushing to bomb shelters after the sirens sounded and the explosions began.

Concern over 'atrocities'

Nato members said reports of Serb atrocities in Kosovo had only strengthened their resolve to continue the air strikes on Yugoslavia.

Nato's Jamie Shea tells of reports of Serb atrocities

Mr Shea said there were reports of intimidation, harassment and widespread violence against the Kosovo Albanian population.

"These are extremely worrying indications," he said.

Other unconfirmed reports of mass killings by Serb forces have been emerging.

Kosovo Liberation Army (KLA) sources were reported as saying on Saturday that Serb paramilitary gangs had slaughtered several hundred Kosovo Albanians in Djakovica overnight.

'Stay the course'

UK Prime Minister Tony Blair: "Air strikes have to continue until we reach our objective"

US President Bill Clinton on Saturday urged Nato forces to "stay the course" and carry on attacking Serb targets.

His comments were echoed by UK Prime Minister Tony Blair, who said Nato air strikes were "the only chance of securing an end to this brutality" in Kosovo.

Nato 'despotism'

Yugoslav President Slobodan Milosevic appeared unbowed as a fourth day of air raids got under way. He said Nato "despotism" must be resisted.

Speaking on state television, President Milosevic said the Nato raids were ruining the United Nations system, and were the worst threat to peace since the Second World War ended in 1945.

Serbian officials said the KLA had stepped up its attacks on Serb forces in Kosovo.