Nato chief Jens Stoltenberg yesterday warned the EU its new defence cooperation pact must not undermine the alliance, as US officials voiced fresh concerns about the flagship initiative.
The European Union launched the scheme with great fanfare in December, touting it as a way to get member states to work more closely post-Brexit and spend more effectively on defence projects.
But the plan has come under fire in recent days from US officials who fear it could draw resources away from Nato and even become a "protectionist" umbrella for European defence manufacturers.
Stoltenberg, speaking on the eve of a meeting of Nato defence ministers in Brussels, said there was "no way" the EU could replace the transatlantic alliance in guaranteeing European security.
Efforts to boost European defence spending were welcome, he said, but only if they were coordinated with Nato plans.
"It will be absolutely without any meaning if Nato and the EU start to compete," the former Norwegian premier told reporters.
"European allies are absolutely aware that the defence, the protection of Europe is dependent on Nato."
The EU's so-called permanent structured cooperation on defence agreement, known as PESCO, has projects in view already to develop new military equipment and improve cooperation and decision-making.
But on Sunday Katie Wheelbarger, a senior official at the US Department of Defense, said Washington regarded some of the proposed initiatives as "pulling resources or capabilities away from Nato".