German Chancellor Angela Merkel and French President Emmanuel Macron agreed in a treaty last month to "contribute to the creation of a European army", drawing stinging criticism from US President Donald Trump.

While NATO supports efforts to get European countries spending more effectively on defence, it is keen to stress that the EU's work must not duplicate or compete with the alliance.

"We need to avoid any perception that Europe can manage without NATO," Stoltenberg said on the eve of a meeting of NATO defence ministers.

"Because two World Wars and a Cold War taught us we need a strong transatlantic bond to preserve peace and stability in Europe."

With Britain, the EU's main military power alongside France, set to leave the bloc on March 29, NATO will become even more important for European defence, Stoltenberg said.

"Especially after Brexit it's obvious that EU efforts cannot replace NATO, because after Brexit 80 percent of NATO's defence expenditure will come from non-EU members."

Trump has repeatedly demanded European NATO countries spend more on their defence but he has mocked the idea of a separate European army.

Stoltenberg has said that done properly, recent EU initiatives to better coordinate and harmonise defence spending could help answer some of Trump's criticisms.

When Merkel and Macron signed their treaty in the German city of Aachen last month, Stoltenberg welcomed it, particularly the emphasis given to international institutions like NATO.