Washington has been rattled by plans for closer EU defence ties and fears the permanent structured co-operation, or PESCO, could diminish NATOs work.
NATO secretary-general Jens Stoltenberg highlighted reservations about PESCO, when he spoke at a security conference in Munich.
He warned Brussels that after Brexit some 80 percent of the alliance’s funding would come from non-EU countries.
And he said the EU’s efforts on defence risked weakening the transatlantic bond, duplicating what NATO was already doing and discriminating against non-EU members of the alliance. He said: “I also highlight that we need to avoid duplication between the Europeans and NATO.
“This is not about an alternative to NATO, this is about strengthening the European pillar within NATO.
“It’s important for Europeans to state again and again that this is not competition for NATO or an alternative to NATO. Some doubts remain.”
Mr Stoltenberg insisted European efforts must match NATO goals for equipment needed to enhance capability, said European assets must be available for all operations and not reserved for particular European ones, and called for “the strongest possible synchronisation”.
He said: “The EU should be clear that it cannot defend Europe by itself.”
The NATO boss’s comments earned a strong rebuke from acting German foreign minister Sigmar Gabriel who warned the US should not stand in Europe’s way.
He said: “No one should try to divide the EU — not Russia, not China, but also not the US.”
Washington’s attention is focused on PESCO which is shaping up to be the EU’s most serious attempt yet at forging closer defence ties.
Of its 28 member states, 25 have signed up to the scheme that involves 17 projects ranging from improving military mobility to developing a new infantry fighting vehicle.
EU officials claim this is simply a response to longstanding calls by Washington for Europe to contribute more to Nato, which have only grown louder since Donald Trump became president.
Mr Trump rattled Brussels with his “America First” rhetoric and his description of NATO as “obsolete”.
European Commission president Jean-Claude Juncker told delegates at the Munich Security Conference: “The United States has been complaining that EU countries do not spend enough on their own military capabilities.
“Now we’re trying to do that, and it’s not right either.”