B-52 long-range bombers with air-launched cruise missiles are being dispatched to Britain

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• NATO head says alliance now united
• U.S. moves B-52s to Europe
• Yugoslav civilians brace for strikes
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BELGRADE, Yugoslavia (CNN) -- With NATO poised to order airstrikes against Yugoslavia, U.S. special envoy Richard Holbrooke ended a marathon session of shuttle diplomacy early Sunday in a last-ditch attempt to defuse the crisis in Kosovo without military action.

The talks were to resume later Sunday, but Holbrooke gave no hint of an impending breakthrough.

"Nothing has changed, and the situation is very serious. ... We certainly don't agree," he told reporters after ending midnight talks with Yugoslav President Slobodan Milosevic and then spending nearly four hours on the phone reporting to Washington.

"On Monday, NATO will meet to authorize action," he said. "We, meanwhile, will continue an intense effort to find a peaceful, acceptable, fully verifiable compliance system, as an alternative."

Holbrooke met with Milosevic in Belgrade Friday and early Saturday, then traveled to Pristina, Kosovo's provincial capital, to meet with ethnic Albanian political leader Ibrahim Rugova. He then traveled back to Belgrade for another round of talks with Milosevic.

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Richard Holbrooke comments on the situation at Belgrade 358K/15 sec. AIFF or WAV sound

At the conclusion of the talks, the Yugoslav government issued a statement through the news agency Tanjung that said, "all necessary conditions exist" for a political solution to the standoff between Milosevic and the West.

Sources close to Milosevic told The Associated Press that the Yugoslav leader has signaled his willingness to comply with most Western demands -- including ending an eight-month military crackdown on ethnic Albanian separatists in Kosovo -- in order to avoid a threatened NATO airstrike.

Milosevic has ruled out the introduction of an international monitoring force into Kosovo, a province inside Serbia, one of the two republics that make up the Yugoslav federation.

The United States has been pushing for such a monitoring force, which would include some NATO troops, to insure that the Yugoslavs live up to any commitments they make.

But Serbian nationalists within Yugoslavia, who consider Kosovo an integral part of their country despite its overwhelming ethnic Albanian majority, view such an international presence as an assault on their sovereignty.

NATO head says alliance now united

Ethnic Albanian refugees from Kosovo are living outdoors

On Saturday in Brussels, ambassadors from the 16 NATO countries met to discuss possible military action. Though officials of some NATO members had earlier expressed concern about airstrikes, Secretary-General Javier Solana said that ambassadors had worked out their differences and were now united in their resolve.

"If Milosevic does not comply, I can tell you that NATO is ready to act," Solana said.

Holbrooke has presented Milosevic with a list of six demands:

0. an end to all violence in Kosovo
1. withdrawal of Serbian security forces and heavy artillery to pre-March levels
2. free access for relief agencies aiding refugees
3. full cooperation with an international tribunal investigating allegations of war crimes
4. repatriation of refugees
5. negotiations on autonomy for Kosovo's ethnic Albanian majority

U.S. moves B-52s to Europe

News Highlights: Gallery: The conflict in review News story archive Yugoslavia's Future: What's next for Yugoslavia Map: Who controls what The Peace Settlement: A guide to the peace plan Map: Serb troop withdrawal The Military Campaign: Strike damage assessment Atlas: NATO and the Balkans Background: Timeline: Trouble in the Balkans A who's who of key players Map: Kosovo and its neighbors A history of the KLA

Even as negotiations continued in Yugoslavia to avoid a military confrontation, NATO continued to make preparations for airstrikes. U.S. Defense Secretary William Cohen ordered six B-52 bombers, a reconnaissance aircraft and 13 tanker planes to deploy to Britain for possible use in any attack.

Albania, which borders Yugoslavia, agreed to allow NATO to use its air space for military operations. Bulgaria was considering a similar request.

Russia, a traditional ally of the Serbs, continues to resist the idea of airstrikes. On Saturday, a top Russian general said his country might end an arms embargo imposed against the Yugoslav federation if NATO bombs fall.

"If the norms of international law are violated, then the ... embargo will also cease to exist for us," said Leonid Ivashov, who heads the department in the Russian defense ministry responsible for international military cooperation.

However, Ivashov does not act as an official spokesman for the Russian military or government, and it was unclear if his views reflected the official position in Moscow.

Yugoslav civilians brace for strikes

Inside Yugoslavia, the civilian population was being prepared for possible airstrikes. Air raid shelters dating from World War II -- many of which now house underground businesses -- were being made ready to house people on six hours notice.

Yugoslav media and nationalist Serb leaders whipped up anti- Western sentiment.

Deputy Prime Minister Vojislav Seselj said the demands being made on Serbs were "really aimed at taking the territory (of Kosovo) from Serbia."

While he said the government was trying to avert a military threat, he said it would not do so "at that cost of sacrificing Serbia or part of Serbian territory."

Correspondents Brent Sadler and Richard Blystone and The Associated Press and Reuters contributed to this report.