In Brussels, Nato ambassadors approved a rare "activation request" for the 16 allied nations to formally commit war planes and other forces for an eventual air campaign against the Serbs to underline revulsion at recent atrocities in Kosovo.
But the Nato council failed to overcome differences among members over whether the alliance needs a clearer mandate from the United Nations before it can attack. Although military readiness is now high, a final political decision is unlikely before next week.
In New York, the Security Council, now under Britain's chairmanship, strongly condemned the atrocities but refrained from calling on Nato to act.
The Security Council is still awaiting a report on Kosovo, due on Monday, from Kofi Annan, the UN Secretary-General. Sources said he appears unlikely to advocate military action since he was unconvinced of their effectiveness.
The response contrasted with expectations raised at the Labour Party conference, where both the Prime Minister, Tony Blair, and the Foreign Secretary, Robin Cook, vowed to punish the Yugoslav President, Slobodan Milosevic, suggesting that the European allies were ready to launch attacks.
The Council's permanent five members remain divided on Kosovo, with Russia and China both opposed to air strikes. Russia yesterday reiterated its opposition to aerial attacks but this stance may be overcome. Senior officials of the so-called Contact Group will meet in London today to discuss the use of force.
Serbia's Deputy Prime Minister, Vojislav Seselj, warned in Belgrade of retaliation against Nato forces "wherever possible". He said: "The West should be aware that their soldiers will be our targets, no matter where they are."
The Independent has also obtained evidence that the current European Union sanctions against Serbia are almost completely ineffective. In a damning indictment, an internal European Commission paper said the sanctions may amount to less than pounds 1m in blocked investments.
In Washington, the State Department warned American citizens to leave Yugoslavia because of the increasing likelihood of military action.