The civilian and military heads of NATO cautioned the European Union on Wednesday against duplicating efforts following remarks by the president of the EU Commission suggesting the bloc should develop an "EU army" to deter Russia.

Jens Stoltenberg, the secretary general of the Western defense alliance, told a news conference he would welcome increased European investment in defense, but said duplication would be inefficient.

"It's important to avoid duplication and I urge Europe to make sure that everything they do is complementary to the NATO alliance," he said.

General Philip Breedlove, the U.S. supreme commander of NATO forces, told the news conference he was "not concerned" by the comments at the weekend by European Commission President Jean-Claude Juncker.

"But what we would like to avoid is any duplication because we need to smartly invest," Breedlove said.

Juncker told a German newspaper on Sunday that the EU needed its own army to face up to a resurgent Russia and other threats as well as to restore the bloc's foreign policy standing around the world.