Diplomatic chief Federica Mogherini insisted there was no question of replacing or competing with NATO, saying the alliance's role was already written into EU treaties.

Senior US officials voiced doubts about the EU plan earlier this week, fearing it could draw resources away from NATO or become a "protectionist" umbrella for European defence manufacturers.

Mogherini said talks with NATO defence ministers including US Defense Secretary Jim Mattis on Wednesday had allowed her to give reassurances that the EU plan did not seek to replace the alliance.

But she dismissed a call made by Mattis on Thursday "to include in written EU documents" that common defence was solely a job for NATO.

"This is already clearly stated in black and white in the EU treaties, so no need further to speculate on that," she told reporters in Sofia, where she was attending a meeting of EU foreign ministers.

"NATO is mentioned in the EU treaty as the responsible organisation for collective defence for those EU member states that are NATO allies, so it's clear the work we're doing on European defence in the EU is not aiming at substituting for NATO when it comes to collective defence."

The European Union launched its so-called permanent structured cooperation on defence agreement, known as PESCO, with much fanfare in December.

The pact, signed by 25 EU members, aims to get EU states to cooperate more closely in spending on defence and developing new military equipment, to avoid waste.

But on Sunday a senior official working with Mattis said Washington had concerns that some of the proposed initiatives risked "pulling resources or capabilities away from NATO".

And on Monday US Ambassador to NATO Kay Bailey Hutchison warned the EU against using PESCO as a protectionist tool, saying there could be serious consequences if it shut US defence companies out of cooperation projects.