Nato Secretary General Javier Solana told reporters the campaign will continue "as long as is necessary to achieve our aims".

And in the United States, President Clinton said the Balkan conflict must be "nipped in the bud" before it spreads into a wider European war.

For the first time, the western alliance hinted that it may send ground troops into Kosovo without a formal ceasefire.

The BBC's Brian Hanrahan: Nato has reaffirmed its faith in bombing

As the Nato foreign ministers met in Brussels, the Serb media reported that a passenger train had been bombed, with the loss of at least nine lives.

Serbian officials said there had been two bombing raids on a bridge over the Morava River - one which had cut power supply lines, and a second which hit two train carriages, setting them on fire.

John Simpson in Belgrade: There do seem to have been some dreadful scenes

Nato acknowledged it had hit a railway bridge at Grdelica and said it was investigating reports that a train carriage had been hit.

Officials described the bridge as an important part of the supply lines for Yugoslav military and security forces in Kosovo.

(Click here for map of latest strikes)

US Secretary of State Madeleine Albright said after the Nato meeting that she was planning to discuss the plight of Kosovo Albanian refugees still in the province with a senior representative of the Kosovo Liberation Army.

The UN Secretary-General Kofi Annan has stepped up his diplomatic efforts to resolve the crisis. He has written to President Milosevic requesting a meeting, and will attend a European Union foreign ministers' meeting on Wednesday.

BBC World Affairs Editor John Simpson: "The tone of the debate wasn't particularly fiery"

Meanwhile, the Yugoslav Parliament has voted unanimously to join a loose union between Belarus and Russia, which has been a strong critic of Nato action.

Ground troops discussed

An American State department spokesman suggested that Nato might put aside its reluctance to use ground troops in a military role in Kosovo.

He said contingency plans for Nato troops to go into Kosovo even without a peace agreement were being studied again.

In London, Ministry of Defence spokesman General Sir Charles Guthrie acknowledged that Nato planners had discussed sending in ground troops.

The BBC's Paul Reynolds: "The emphasis on the use of ground troops has changed somewhat"

But Sir Charles said there was no current plan to use ground forces.

He said that poor weather on Sunday had hampered missions.

But he added that RAF planes had, for the first time in the campaign, engaged targets through cloud.

Correspondents say ground troops were not on the agenda at the Nato meeting in Brussels.

The talks were aimed at examining how to help the hundreds of thousands of Kosovo Albanians refugees, as well as a long-term strategy for the stability of the Balkans.

US Secretary of State Madeleine Albright is to meet a KLA representative to hear reports of what is happening inside Kosovo.

She said: "I am meeting ... because we are constantly trying to get information about what is happening on the ground in Kosovo."

She said: "Belgrade is taking every opportunity to make a bad situation worse.

"We are concerned that as many as 700,000 people are at risk inside Kosovo. If those people are allowed to die we will hold President Milosevic responsible."

She added that the Nato meeting also discussed the longer term future and efforts to ensure Nato would not be called on again in the region.

And French Defence Minister Alain Richard said it was possible that a future peacekeeping force for Kosovo may not be under Nato's direct control.

At Rambouillet, Yugoslavia rejected all demands for a Nato-led force in Kosovo.

Some Nato governments, including the Italians and Greeks, have concerns about the use of force but are still supporting the camapign.

George Eykyn reports: "Nato's credibility is on the line"

BBC Defence Correspondent Jonathan Marcus said Nato's main political aim remains the introduction of an international military force to facilitate the return of refugees.

Mrs Albright is due to meet Russian Foreign Minister Igor Ivanov in Oslo on Tuesday in an effort to avoid a further deterioration in relations on the Kosovo issue.

Later in the week, European Union heads of government are also expected to discuss Kosovo.

Hungary, a new member of Nato, has resolved its diplomatic row with Russia over an aid convoy.

Russia accused Hungary of obstructing the convoy because of loyalty to Nato.

Hungary held up the aid trucks for two days before deciding only to ban nine vehicles - five with armoured cabins and four tankers carrying diesel - from the humanitarian drive to Serbia.

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