Serb media and witnesses described the overnight attacks on Nis - Yugoslavia's third city - as the heaviest of the campaign.

The city's mayor told Reuters news agency that Nato had dropped cluster bombs on the main hospital and market area, killing at least six people.

The BBC's Brian Hanrahan: "Suddenly there are signs of a change"

The Yugoslav Foreign Ministry put the figure at 10 dead and 50 injured.

City officials also said flames could be seen rising from the city's airport.

Nato spokesman Major General Walter Jertz of the German Army said Nato forces had not targeted a civilian hospital but it is checking reports.

Other targets on the 44th night of Nato's air offensive included a bridge on the main railway line from Belgrade to Bucharest, and targets in the second city, Novi Sad.

(Click here to see a map of last night's Nato strikes)

The strikes came as the United States announced it was sending a new wave of aircraft to join the campaign against Yugoslavia.

Defence Secretary William Cohen ordered the deployment of another 176 planes to Europe, bringing to more than 800 the number of US aircraft available for use by Nato.

The BBC's Mike Williams: "Nato made it clear there is to be no pause in the bombing"

The US House of Representatives has approved more than $13bn in extra funds for the Yugoslav campaign, twice the amount requested by President Clinton last month. The Senate has yet to vote on the spending commitment.

Cautious reaction

Yugoslavia has reacted cautiously to the draft peace plan agreed by Russia and the world's leading industrial countries on Thursday.

Foreign Ministry spokesman Milisav Paic said his government was willing to accept a UN presence in Kosovo but that its mandate and composition should be negotiated between Belgrade and the UN.

Yugoslav UN Ambassador Vladislav Jovanovic: "Many countries support our right to defend our country"

Yugoslavia has agreed to allow a team of UN humanitarian officials to visit Kosovo, after a request from the UN Secretary General, Kofi Annan.

The UN said an advance team of officials would leave for Belgrade in the next few days.

The draft peace plan agreed in Bonn on Thursday envisages the deployment of "effective international civil and security presences" endorsed by the United Nations, to secure Kosovo after the withdrawal of Serb forces.

It provides for the return of refugees to their homes and the establishment of an administration approved by the UN Security Council.

Diplomats are due to start work on the details of the agreement in Bonn on Friday, before it is sent to the United Nations.

No change of strategy

President Clinton: Need to stand firm

President Clinton said the shift in Russia's position to supporting the eventual deployment of a United Nations force and self-government in Kosovo showed a real peace process was underway.

But, speaking in Germany, he said the chance for a successful end to the conflict depended on Nato firmness, and there was no reason to change the bombing strategy.

Russia - Serbia's traditional ally - had been coming under increasing pressure to agree a framework to end the conflict.

BBC Correspondent Caroline Wyatt said the agreement suggested Nato forces might be deployed in Kosovo but not in a leading role.

Caroline Wyatt in Bonn: "No specific mention of Nato's role in peacekeeping force"

Russian Foreign Minister Igor Ivanov said Nato could not participate in an international security presence in Kosovo without Yugoslavia's agreement.

US Secretary of State Madeleine Albright reiterated that Nato forces - including US troops - must be at the core of any international force deployed.

Rugova security call

The Kosovo Albanian political leader, Ibrahim Rugova, has said an international peace force for Kosovo - comprising troops from Nato and other countries - is essential if refugees are to return home safely.

Speaking for the first time since unexpectedly arriving in Italy on Wednesday, Mr Rugova said Serb troops also had to withdraw from the province, but he did not comment on whether he supported Nato's military action.

He did not say whether he had been held under house arrest by Serb forces, as had been reported.

Border re-opened

Macedonia says it is having difficulties absorbing the refugee influx

The Former Yugoslav Republic of Macedonia has said its Blace border with Kosovo has been re-opened and refugees are being allowed into the country again.

Government officials are quoted as saying that the decision to close the border for a few hours on Wednesday was to draw attention to Macedonia's difficulties in dealing with the huge influx of Kosovo Albanians in the past few weeks.

• Analysis: Is this the endgame?
• Macedonia border deserted
• Rugova calls for peace force
• Belgrade allows UN Kosovo mission
• Kosovo Albanians: Who's left?