The United States vetoed a six-month extension of the United Nations peacekeeping mission in Bosnia today because the Security Council refused to grant the small contingent of Americans serving there immunity from the world's first permanent war crimes court.

But the United States subsequently agreed to a European request to allow the mission to continue operations for three more days, raising a slim chance for a compromise before the mission is forced to cease operations Thursday.

U.S. and European officials said Washington would use the three days to increase pressure on foreign leaders to meet its demands.

The move marked a dramatic escalation in the Bush administration's effort to place U.S. citizens beyond the reach of the International Criminal Court, which comes into existence Monday despite fierce U.S. opposition.

It also cast fresh uncertainty over the long-term fate of U.N. peacekeeping and the more immediate future of the United Nations in Bosnia, where a force of more than 1,500 U.N. police, including 46 Americans, was preparing to gradually relinquish its responsibilities over the next six months to a mission run by the European Union. The U.N. Mission in Bosnia-Herzegovina and a NATO force have served as the guarantors of stability since the country's bloody war ended in 1995.

John D. Negroponte, the U.S. ambassador to the United Nations, voted to block a resolution supported by 13 members of the 15-nation council that would have extended by six months the Bosnia mission, whose mandate expired at midnight. Bulgaria, one of nine countries that sponsored the resolution, abstained.

Negroponte said he had vetoed the resolution "with great reluctance" and pledged that the United States would "stand by" its commitment to pursue peace and stability in Bosnia. "The fact that we are vetoing this resolution in the face of that commitment, however, is an indication of just how serious our concerns remain about the risks to our peacekeepers."

He said the United States would pull out three military observers serving with the U.N. in East Timor if it did not resolve the dispute over the ICC in the coming week. Another U.S. official said Washington was prepared to accelerate the withdrawal of the 46 Americans in Bosnia.

It remained unclear what impact today's decision would have on a much larger NATO peacekeeping force in Bosnia, which was established under the U.S.-brokered 1995 Dayton Accords. U.S. and European officials said Washington is exploring ways to determine whether the NATO force in Bosnia, which includes a contingent of about 3,100 U.S. troops, can remain in place.

The tough U.S. negotiating tactics infuriated court advocates, including Washington's closest allies, who characterized the U.S. veto as an extraordinary, and unnecessary, attempt to use the council, whose resolutions are legally binding, to amend a global treaty that has broad international support.

"This isn't about the vulnerability of Americans deployed in peacekeeping operations," said Richard Dicker, a court proponent at the New York-based Human Rights Watch. "The United States is using the Security Council as a battering ram against the integrity of the treaty."

The standoff in the council dampened what court advocates had hoped would be a celebration of the court's birth. The treaty creating the criminal court has been signed by 138 countries and ratified by 74. The ICC was established to prosecute alleged dictators and war criminals for the most serious crimes, including genocide, war crimes and crimes against humanity.

Although the Clinton administration signed the treaty in December 2000, the Bush administration renounced it in May, arguing that the tribunal might conduct politically motivated trials against Americans.

U.S. officials acknowledged that American peacekeepers' exposure to prosecution by the court is limited. There are 677 American police, 34 military observers and one peacekeeper serving in the U.N.'s 15 peacekeeping missions. U.S. military forces serving in U.N.-approved missions already have immunity from prosecution and arrest by local authorities.

But the Bush administration insists it needs explicit protections for all current and former American nationals from the ICC, which will be based in The Hague.

France's and Britain's envoys said that U.S. police and peacekeepers had little to fear from an international court and that Washington could obtain adequate assurances of immunity through provisions in the ICC treaty, including one that would allow the United States to negotiate bilateral agreements with countries that host American forces.

"While we understand the United States' concerns regarding the court, we do not share them," said Jeremy Greenstock, Britain's U.N. ambassador. "We believe that the risk of peacekeeping personnel appearing before the court is extremely small."

France's U.N. ambassador, Jean-David Levitte, urged Washington to withdraw a small unit of American citizens, which includes the U.N.'s top official, Jacques Klein, and 46 American police officers rather than shut down the Bosnia mission.

"What is at stake is the very capacity of the United Nations to continue peacekeeping operations," Levitte said. "For the United States the simplest thing is to withdraw the 46 U.S. police. . . . There is simply no need to kill off UNMIBH."

U.N. Secretary General Kofi Annan raised concern over the possibility that Washington could sacrifice some of the U.N.'s 14 other peacekeeping operations when their mandates come before the council for renewal. And he warned that the abrupt closure of the U.N. mission in Bosnia would "severely compromise" the transfer of responsibility to the European Union. "The [Bosnian] state and its institutions are still fragile and under pressure from nationalist forces," Annan said. "Unless an agreement can be reached on an orderly wind-down of the mission, the police in Bosnia and Herzegovina will be left unmonitored, unguided and unassisted."