Just what kind of shape the EU defense project will have is undecided. That has to be worked out in the ongoing negotiations over the bloc's first constitution.

But the United States is concerned that some European states, notably France and Germany, want the union to have a military headquarters that would be independent of NATO.

At a meeting of NATO ambassadors Monday, alliance officials say the Europeans tried to assure the United States that a European military project would complement NATO and not undermine it.

NATO spokesman Jamie Shea said that whatever the European Union decides to do in the future, he expects it to be fully compatible with NATO. "The European allies of the United States assured Washington that the plans to build up a common foreign and security policy by the European Union, designed to strengthen NATO, to complement it too, allow Europe to be a more weighted partner of the United States, and not to undermine NATO," he said.

Mr. Shea said the Europeans pledged to keep the United States and other non-EU allies like Canada and Turkey informed of their defense plans.

At a European summit last week, France and Germany backed away from their original plan for a separate EU military headquarters. But they sought and received British support for a mechanism that would grant the European Union some form of autonomous military planning capability.

British Prime Minister Tony Blair says the European Union needs to be able to act in cases where the United States and NATO do not want to get involved.

Though it says it favors a stronger European defense capability, the United States argues that the European Union needs to spend more on state-of-the-art weapons and other military resources, rather than on a headquarters and a military bureaucracy separate from NATO.

One EU diplomat says remarks by the U.S. ambassador to NATO last week that the EU defense plans are a threat to the future of the alliance underscore concern in Washington over what U.S. officials perceive to be Britain's cozying up to the Franco-German position on an independent EU operational planning mechanism.

EU foreign policy chief Javier Solana, a former NATO secretary-general, is trying to play down transatlantic differences over the issue. "The relations between the European Union and NATO are basic relations, and nobody wants to question that, and nobody wants to damage that profound relationship," he said.