In a stunning setback for the United States that could require the Bush administration to alter part of its war plans, the Turkish parliament rejected by three votes tonight a motion that would have allowed U.S. troops to use Turkish bases to open a northern front against Iraq.

The leadership of the governing Justice and Development Party, which had endorsed the U.S. deployment, could seek another vote when parliament reconvenes next week. But its failure to win approval of the resolution tonight, despite a sizable legislative majority and weeks of preparation, raised questions about its ability to hold on to control of the government if it tried and failed again to authorize the U.S. deployment.

The Pentagon has been waiting for weeks for permission to begin moving its forces toward Turkey's 218-mile border with Iraq, and warned last week that it was running out of time to decide whether an armada of U.S. ships carrying tanks and equipment, including several cargo vessels already in Turkish waters in the eastern Mediterranean, should change course and sail through the Suez Canal to Kuwait.

A senior administration official in Washington described the Turkish vote as "absolutely a big deal," and said the administration must now decide whether to wait and see what the Turkish government does next, or immediately move forward with a backup plan to deploy the Army's 4th Infantry Division to Kuwait instead of Turkey.

The defeat in the Turkish parliament was another setback in U.S efforts to win international support for a war against Iraq. The U.S. push was further complicated today as Iraq began destroying its banned Al Samoud-2 missiles under the supervision of U.N. inspectors.

The Justice and Development Party, which took power in elections less than four months ago, holds 362 seats in the 550-member parliament, but managed to muster only 264 votes in favor of the U.S. deployment. The unexpected defection of more than a quarter of its deputies and a unified stand by the opposition resulted in 250 votes against the proposal and 19 abstentions. The other 17 legislators did not attend the session.

The closeness of the vote, which took place in a closed session after a three-hour debate, threw the legislature into confusion for several minutes before the parliamentary speaker, Bulent Arinc, ruled that the government's motion had failed because it had not won a majority from the 533 legislators present.

Prime Minister Abdullah Gul, his face drawn and weary, told reporters after the vote that the party was evaluating its options, but he refused to say whether it would pursue another vote. "We all have to respect the decision of the parliament. This is the requirement of democracy," he said. "We will evaluate all this as a party and a government, and do whatever is required. The government is no doubt aware we are in a critical period."

Recep Tayyip Erdogan, leader of the ruling party, described the parliament's decision as "a completely democratic result." "What more do you want?" the Anatolian news agency quoted him as saying after a meeting with party leaders. "May it be for the best."

A senior member of the government, who asked not to be identified, said Erdogan has called an emergency meeting of the party's governing board for Sunday. "We haven't come up with a strategy yet," the official said. "We will either renew the resolution and try again, or we will say this is the view of the Turkish people, and we will not be able to take foreign soldiers on Turkish soil. We'll be talking about it very carefully."

The official blamed the outcome of the vote in part on the U.S. government's refusal to grant more concessions in talks over what economic aid and political assurances Turkey would receive in exchange for allowing its territory to be used as a staging area against Iraq. Negotiators are near a deal involving guarantees about the future of Kurdish-controlled northern Iraq and $6 billion in grants, which could be leveraged into as much as $24 billion in loans, but the agreement has not been finalized.

U.S. officials, including Secretary of State Colin L. Powell, had pressed the Turkish government to go ahead with a vote. "It's hard to speculate about what happened," the senior Turkish official said, "but it's partly because the Americans have not come up with a sound agreement that could be signed by everyone and then presented to the parliament."

Other analysts argued that the Justice and Development Party had been outmaneuvered by a Turkish political establishment -- including the military, government bureaucracy and other state institutions -- that is wary of the new party because of its popular appeal and roots in political Islam. These institutions normally might have backed a U.S. deployment, the analysts said, but instead have remained silent and sought to use the issue to weaken the party.

"Do not take part in this disgusting war. Do not get crushed under the weight of this misery," one opposition lawmaker, Onder Sav, said on the floor of the parliament. "If only one soldier's corpse must be carried on shoulders [to a funeral], then we will not forgive you. The voters will not forgive you either."

Turkey's cooperation is important to the Pentagon's plan to divide the forces of Iraqi President Saddam Hussein by simultaneously attacking from the south, through Kuwait, and from the north, across the Turkish border. Troops based in Turkey would play a critical role in securing oil fields in northern Iraq and keeping the peace between Kurdish factions, and between Kurdish and Turkish troops.

If the 4th Infantry Division is diverted to Kuwait, the U.S. military is likely instead to build any attack from the north around lighter forces that are more easily deployed by air. But the loss of Turkish bases could also make it more difficult to resupply U.S. forces with ammunition, food and fuel.

Speaking after a meeting at the Foreign Ministry tonight, the U.S. ambassador to Turkey, Robert Pearson, expressed disappointment in the vote but said the United States would continue to consult with the Turkish government. Months of delays and tough bargaining by Turkish leaders have already strained relations between the United States and Turkey, a historic ally and a member of NATO.

Fehmi Koru, a political analyst who writes a column for the Yeni Safak newspaper, predicted that the government would not risk submitting the resolution for another vote, in part because so many members of the cabinet had opposed the U.S. deployment. Though the party leadership failed by only three votes, "if they tried again, and they get the same outcome, it would mean the end of this government for sure," he said. "They've already seen almost half the parliament vote no."

Opposition leader Deniz Baykal urged the government to abandon plans for a U.S. deployment. "I hope the government will respect the will of the parliament," he said. "This situation shows Turkey is not willing to join this war."

Turkish leaders have only recently begun trying to convince the public that their country might benefit by cooperating in a U.S.-led war against Iraq, by preventing the emergence of an independent Kurdish state in northern Iraq and restoring trade with a friendly Iraq freed from international sanctions.

But public opinion in this predominantly Muslim nation of 67 million people remains strongly against a war. Many people fear it would be a repeat of the Persian Gulf War of 1991, when Turkey was swamped with half a million refugees and its economy was devastated, ushering in a decade of financial instability that culminated in 2000 with a crisis in which the currency collapsed and unemployment skyrocketed.

Even as lawmakers debated the U.S. deployment, tens of thousands of antiwar protesters massed in the streets of the Turkish capital, their path to the parliament building blocked by riot police and armored cars.

Many demonstrators said they expected their government to ignore them and give in to the U.S. request.

"The Turkish people are against a war, but the government won't listen to us," said Yasemin Karaoglan, 24, a graduate student among the crowd of chanting, singing protesters. "They can't resist the power of the Americans."

Staff writer Karen DeYoung in Washington contributed to this report.

A member of the Turkish parliament holds a sign saying "No to War" before a vote on allowing U.S. troops to deploy on Turkish soil.