And Washington is seeking clarification from its European partners at a special meeting next week, NATO sources said.
"What the US wants is to ask the EU what is happening," said a NATO source, confirming that the ambassadors from NATO's decision-making body will meet on Monday to prepare for a formal NATO-EU diplomatic gathering on Tuesday.
The Tuesday meeting will discuss the "state of play" of NATO-EU relations, said the source.
The row emerged as British Prime Minister Tony Blair held four-way talks on defence and other issues with his partners from France, Germany and Belgium -- who agreed controversial plans for a European military planning headquarters earlier this year.
A number of NATO delegations protested after US envoy Nicholas Burns warned at a closed-door meeting of alliance ambassadors Wednesday that EU initiatives represented the "most significant threat to NATO's future".
He was referring to plans unveiled in April by four countries that opposed the Iraq war -- Belgium, France, Germany and Luxembourg -- for a planning headquarters for autonomous EU military operations.
One diplomat said the meeting turned "hot" after the comment, the latest attack on the plans by the "gang of four".
"The tone (of the comments) was taken badly by several delegations," said the source, indicating that the US envoy may have intended his remarks as a warning to Britain.
Britain, the United States' closest ally, has also criticized the headquarters plan, although there have been signs of compromise from London in recent weeks.
On the sidelines of an EU summit in Brussels Thursday, Blair held talks lasting 45 minutes with French President Jacques Chirac, German Chancellor Gerhard Schroeder and Belgian Prime Minister Guy Verhofstadt, officials said.
"We are working together to allow Europe's defence to evolve," Chirac's spokeswoman Catherine Colonna told reporters.
At a summit in Berlin with Chirac and Schroeder last month, Blair reportedly conceded that the EU should have its own military planning capacity without having to resort to NATO.
According to the Financial Times newspaper on Thursday, Washington has reacted with dismay to "mixed signals" from Blair's office about the EU defence plans.
It said US national security adviser Condoleezza Rice had telephoned Nigel Scheinwald, Blair's chief foreign policy aide and formerly Britain's ambassador to the EU, to express her concern.
But Britain, while staying open to ideas to strengthen the 15-nation bloc's defence cooperation, reaffirmed its opposition to the EU military headquarters project.
Foreign Secretary Jack Straw told reporters in Brussels that the EU was right to discuss boosting its own defence capacities and planning resources.
But he added: "What we intend to make sure is that nothing undermines the pre-eminence of NATO as the guarantor of the territorial defence of Europe."
Nothing should "undermine the way in which European security and defence policy fits in and complements and doesn't get underneath NATO's prime role", Straw said.
"We have profound concerns about that."
Polish Prime Minister Leszek Miller, who has committed troops to the US-led occupation in Iraq, said freelance military ventures by the EU risked the "destruction" of NATO.
"We are against anything that risks creating a rival to NATO," he said in Brussels.