More than 100 nations hailed the birth of the world's first International Criminal Court on Monday as a landmark for global justice, vowing that its mission to prosecute and deter future war criminals will not be sabotaged by U.S. opposition.

The new court's main targets are the future Pol Pots and Adolf Hitlers of the world. It will prosecute those responsible for genocide, crimes against humanity and war crimes on or after July 1.

The United States has opposed the court, fearing it could go after Americans in frivolous political prosecutions.

At the start of the final two-week meeting of the commission that has been preparing for the court's operation in The Hague, Netherlands, speakers from all continents hailed the historic entry into force of a 1998 Rome treaty establishing the court.

Many called it the greatest advance in international law since the Nuremberg and Tokyo trials for World War II's German and Japanese war criminals.

"The past century witnessed the worst crimes in the history of mankind," said Denmark's U.N. Ambassador Ellen Margrethe Loj, speaking on behalf of the European Union.

"Yet few perpetrators have been brought to justice. Let us together establish a credible, fair and effective International Criminal Court which will serve as a deterrent — as a signal that impunity will no longer be tolerated," she said.

The Rome treaty won its 75th ratification Monday from Australia. It now has 139 signatures — including the United States. Former President Clinton signed the treaty but the Bush administration announced in May that it wants nothing to do with the court. The U.S. seat was empty Monday.

Standing alone, and against its closest allies, the United States is demanding immunity from the court for American peacekeepers — and is threatening to end the 1,500-strong U.N. police training mission in Bosnia at midnight Wednesday otherwise.

The United States has also warned that all U.N. peacekeeping is at stake, which could have serious ramifications from the Middle East to Africa, Cyprus and Afghanistan. Mandates for four U.N. peacekeeping missions — in Lebanon, Georgia, Western Sahara and the Croatian enclave of Prevlaka — expire this month.

To emphasize its demand, the United States dramatically vetoed a Security Council resolution Sunday night to extend the Bosnian police mission as well as authorization for the 18,000-strong NATO-led force in the country.

But two hours later, Washington agreed to a 72-hour extension to give more time for talks — and for preparations to shut down the mission if necessary.

U.N. Secretary-General Kofi Annan, concerned about the threat to U.N. peacekeeping, urged all parties to try to find a solution before the Bosnian mandate expires.

"We are very concerned about what will happen in Bosnia if the mission has to be abruptly terminated," U.N. spokesman Fred Eckhard said. "It is hard to imagine that the mission would not be substantially damaged if it had to be shut down at midnight Wednesday."

NATO said its Bosnian force operates under the 1995 Dayton peace agreement that ended the country's 3-year war and will not be affected.

Diplomats said contacts were taking place between capitals, but no movement was reported between the United States and the 14 other council members who overwhelmingly support the court.

The Security Council was scheduled to meet Tuesday to discuss the Bosnia mandate and the U.S. demand.

Court supporters accused the Bush administration of holding U.N. peacekeeping hostage to its demand for immunity.

"The United States is using the peacekeeping mandate's renewal for Bosnia as an opportunity to launch a major war against the Rome statute," said William Pace, who heads the Coalition for an International Court, a group comprised of more than 1,000 organizations that support the tribunal.

If fear of prosecution for American peacekeepers was the only issue, he said, the United States could simply withdraw from all missions instead of threatening to end them.

"The issue is really not connected to peacekeeping. It's an ideological opposition by ... the extreme conservatives in the Department of Defense, the Congress and the Bush administration ... against establishing any international court that would have jurisdiction over U.S. nationals that the United States has not ratified," Pace said.

White House spokesman Ari Fleischer said the Bush administration "strongly supports" Bosnian peacekeeping but that the treaty threatens to overreach and ensnare American diplomats and military personnel on overseas duty.

"This is a very important matter of principle about protecting Americans who uniquely serve around the globe in peacekeeping efforts," he said. "The world should make no mistake: The United States will stand strong and stand on principle to do what's right to protect our citizens."

Canada's Ambassador to Sweden Philippe Kirsch, who chairs the meeting, called the U.S. veto "unwarranted," stressing that the treaty contains many safeguards — first and foremost that the court will not step in unless countries are unwilling or unable to prosecute their nationals.

Former U.S. Ambassador Richard Holbrooke said he saw no danger to Americans in the peacekeeping forces in Bosnia.

"To jeopardize this force in this way is unnecessary," he said in an interview on Norwegian state radio NRK. "I want to point out that if this continues, it will affect not only Bosnia but Kosovo, Africa, East Timor and most importantly Afghanistan."