Brussels must be careful not to weaken or undermine the North Atlantic Treaty Organization (NATO) as it builds a supranational European Union (EU) Army, American military leaders have told their EU allies.

Last November, 25 EU states signed the Permanent Structured Cooperation on Security and Defence (PESCO), agreeing on a multi-billion-euro common defence fund and a shared military headquarters.

The European Commission’s president then demanded a “fully-fledged Defence Union” by 2025 before the bloc’s foreign minister revealed they were working at “full speed” to create a unified “defence force” on a “continental scale”.

U.S. leaders have now raised concerns that the EU Army could weaken the NATO defence alliance and block American military manufacturers from bidding on certain European projects at the Munich Security Conference on the weekend.

Aide to Defense Secretary Jim Mattis, Katie Wheelbarger, said: “We are supportive of [PESCO], as long as it is complementary to and not distracting from NATO’s activities and requirements.”

“We don’t want to see EU efforts pulling requirements or forces away from NATO and into the EU,” Ms. Wheelbarger added.

In addition, the American ambassador to NATO, Kay Bailey Hutchison, said Washington did not want PESCO “to be a protectionist vehicle for EU,” according to The New York Times.

“We’re going to watch carefully, because if that becomes the case, then it could splinter the strong security alliance that we have,” she added, referring to NATO.

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NATO Secretary General Jens Stoltenberg has also raised his concerns, insisting it would make no sense for “NATO and the EU to start to compete” and saying there is “no way” PESCO could take over the NATO’s role in safeguarding Europe.

“It’s important for Europeans to state again and again that this is not competition for NATO or an alternative to NATO,” he added at the conference in Munich. “Some doubts remain.”

He also said there should be “the strongest possible synchronisation” with NATO members who are not members of the bloc, “because the EU cannot defend Europe by itself”.

Sigmar Gabriel, the acting German Foreign Minister, hit back as Mr. Stoltenberg’s comments, warning the U.S. not to stand in way of an EU Army.

“No one should try to divide the EU – not Russia, not China, but also not the U.S.,” he told the conference.