NATO chief Jens Stoltenberg on Tuesday 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."
- 'Protectionist' warning -
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 was concerned some of the proposed initiatives risked "pulling resources or capabilities away from NATO".
And the US stepped up its criticism on Monday, with Ambassador to NATO Kay Bailey Hutchison warning the EU there could be serious consequences if it shut US defence companies out of cooperation projects.
"Certainly we do not want this to be a protectionist vehicle for the EU and we're going to watch carefully, because if that becomes the case then it could splinter the strong security alliance that we have," she told reporters.
"We want the Europeans to have capabilities and strength but not to fence off American products or Norwegian products or potentially UK products."
Britain, which is leaving the EU next year, is one of only three member states along with Denmark and Malta not to sign up to the pact.
Under the rules of the cooperation agreement, as a non-EU state Britain will be able to take part in specific missions but will have no decision-making role.
The disquiet over PESCO means a working dinner at the defence ministers' meeting on Wednesday, which EU diplomatic chief Federica Mogherini is set to attend, takes on added importance.
A European diplomat insisted EU defence cooperation posed no threat to NATO, saying that while "a little explanatory work" was needed to reassure the Americans, there was no fundamental conflict.
But the diplomat said the fact the US does not have a full ambassador to the EU in post was a problem.
"If the US appointed an ambassador to the EU -- which they haven't done for over a year -- it would be easier to explain what the Europeans are doing," the diplomat said.