Turkey's parliament has narrowly failed to approve the deployment of US troops on its territory for a possible war with neighbouring Iraq.
US troops are waiting on ships off Turkey's southern coast
MPs voted 264-250 in favour of the deployment, but the motion fell four votes short of the required majority of deputies present in the chamber.
The vote came amid mounting pressure from Washington, which has ships laden with tanks anchored off the Turkish shore.
In its first reaction, the State Department asked for "clarification" of the Turkish vote.
Turkey will receive $15bn in aid and loans from the US if it allows the deployment.
The motion - if passed in a subsequent vote scheduled on Tuesday - would also authorise the government to send Turkish troops to Kurdish-populated northern Iraq in the event of war.
The US urgently wants to deploy 62,000 troops and more than 250 planes in Turkey as part of its military plans.
Turkey could send twice as many troops to northern Iraq.
The BBC's Jonny Dymond in Ankara says the knife-edge vote is a massive blow for the four-month-old Turkish Government which has a massive majority in parliament.
But he says, it is in accord with overwhelming popular disapproval of a war against Iraq - thousands took to the streets as the vote was being taken.
More than 50 deputies disobeyed the leaders of the Prime Minister Abdullah Gul's governing Justice and Development party.
The party's leaders are due to meet on Sunday to discuss what to do next.
No one expected the vote to be so close, our correspondent says, and no-one knows what will come next.
The Turkish Government now faces the prospect of either conceding its loss and forfeiting the deal that it worked out so painfully with the United States or calling another vote in the hope that some of its own MPs might change their minds.
Opinion polls show that 80% of Turks are opposed to the war and tens of thousands of protesters, from academics to family parties, turned out in central Ankara.
They chanted "No War" and "We don't want to be America's soldiers".
Turkey, the only Muslim state in Nato, is also afraid of alienating a key ally.
The military deal agreed by Washington and Ankara is believed to cover both the practicalities of how US troops will operate within Turkey - including which country's laws they are subject to - and the much more sensitive issue of how the Turkish and US armies would co-operate in Iraq.
In other developments:
- Iraq destroys four banned al-Samoud II missiles, meeting a UN deadline
- The United Arab Emirates proposes to the Arab League summit in Egypt that Saddam Hussein and fellow leaders should go into exile to avert war
- UN weapons inspectors conduct their first private interviews with Iraqi scientists for three weeks; one was a biologist, another an engineer
- Pope John Paul II has written a message to President Bush outlining his concerns about a possible war in Iraq, and will send a senior cardinal to Washington in the next few days to deliver the letter.