The news coincided with the resumption of diplomatic efforts to try to find a basis for an international settlement to the crisis.

For its part, the United States has said it intends to free two Yugoslav prisoners of war held at a US base in Germany.

Doubts over air war

The assessment of a possible ground deployment was announced after senior US military leaders raised doubts over the effectiveness of the air campaign.

BBC Diplomatic Correspondent Bridget Kendell: "Forces in Kosovo are preparing for the worst"

Newsweek magazine reported that US military chiefs had told the US Government that it could not win the conflict with Yugoslavia using air power alone.

The former Chairman of the US Joint Chiefs of Staff, General Colin Powell, also said the alliance should never have ruled out the use of ground troops.

The BBC's David Shukman: "The movement of troops maybe too little, too late"

In a television interview, he said Nato had to be prepared to suffer casualties if it was serious about achieving its goals.

Nato said on Monday that it had carried out 22,246 sorties since the campaign began, including 5,970 strike sorties. Its attacks on Sunday night were hampered by bad weather.

Ground option

Nato Secretary-General Javier Solana is to determine a date on which troops could safely enter the disputed territory.

The BBC's Ben Brown reports on recent KLA activity

UK Foreign Secretary Robin Cook said Mr Solana had been asked to advise the allies on how long the Yugoslav army could continue effective resistance.

Mr Cook insisted that a formal agreement from Yugoslav President Slobodan Milosevic was not a pre-condition for a ground operation. He also dismissed suggestions of any rift in the Nato alliance.

However he said. "We are not going to launch a major invasion against organised armed resistance."

Mr Cook also said Nato had documented some 80 incidents of Serb forces using Kosovo Albanians as human shields.

'No progress on G8 plan'

On the diplomatic front, Russian Foreign Minister Igor Ivanov met his European Union counterparts in Brussels to discuss ideas for bringing long-term stability to the Balkans once the conflict is over.

The BBC's Peter Biles: "The search for a new diplomatic solution has taken a new turn"

Moscow said it did not expect the meeting to produce any progress in implementing the G8 principles for a Kosovo settlement.

The plan - agreed by the G7 industrial nations and Russia - has been undermined by Nato's bombing of the Chinese embassy in Belgrade nine days ago.

Russia and China have been putting pressure on Nato to end its bombing campaign. Their status as permanent members of the UN Security Council means that any eventual UN resolution on Kosovo will need their assent.

However, Spanish Prime Minister Jose Maria Aznar has said a diplomatic breakthrough would be possible in the next few days. He was speaking after talks in Moscow with Russia's Balkans envoy Viktor Chernomyrdin.

Schröder presses for unity

German Chancellor Gerhard Schröder - who is current chairman of the EU heads of government - is due to meet Italian Prime Minister Massimo d'Alema, in an attempt to shore up apparent divisions within the EU over Kosovo.

Rome has broken ranks with its fellow Nato members by saying that the bombing should end as soon as a UN resolution is agreed with Russia and China.

Earlier on Monday, Mr Schröder said the European Union backed Finnish President Martti Ahtisaari as an international mediator in the Kosovo conflict.

Mr Schröder faces domestic tensions over Kosovo, with leading members of the Green Party - the junior partner in Germany's governing coalition - pressing for Nato to heed their recent resolution demanding an immediate halt to the bombing.

UN team to view Kosovo

The UN mission will assess the situation of displaced people in Kosovo

A United Nations team is in Yugoslavia to assess humanitarian needs in Kosovo.

The inspection, which is to take place with the permission of President Milosevic, will be the first since the start of the bombing and the refugee crisis.

The team, comprising 16 people from UN agencies and the British Save the Children Fund, arrived in Belgrade on Sunday.

"We are worried a bout the hundreds of thousands that are said to be on the move in that province," team leader Sergio de Mello said.

Weather halts bombing

Poor weather caused Nato to cancel 75% of its planned strike sorties on Sunday, the 54th day of Operation Allied Force.

(Click here to see a map of the most recent Nato strikes)

The attacks which did go ahead included strikes against military vehicles and troops in the open. There was also an attack on a radio relay site, a bridge and two airfields.

Nato said better weather - expected to last at least until Thursday - would allow more intensified operations to be carried out.

• Analysis: Waiting for the Apaches
• Freedom in sight for Serb PoWs
• Embassy blame shifts to map agency
• Blair on Balkan solidarity mission

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