NATO Secretary General Jens Stoltenberg warned Thursday that the European Union cannot defend its citizens alone without the help of the trans-Atlantic alliance.

Some EU leaders have been pushing for their union to develop more "strategic autonomy", a move which some see as setting up a rivalry between Brussels and Washington.

But Stoltenberg, a guest at the College of Europe, said that EU member states that were members of NATO made up only a fifth of the total alliance defence spending that protects European shores.

"A European Union that spends more on defence, invests in new capabilities and reduces the fragmentation of the defence industry, is not only good for European security," he said.

"It is also good for trans-Atlantic security," he said, appearing with the college's director, former EU foreign policy chief Federica Mogherini.

"But the EU cannot defend Europe alone. More than 90 percent of EU citizens live in a NATO country. But EU members provide only 20 percent of NATO's defence spending."

And he noted that, while 21 of the members of the European Union are also members of NATO, their exposed flanks are often defended by bigger-spending non-EU allies.

"This is not only about money. It is also about geography," he said, while stressing the close cooperation between the two Brussels-based institutions. 

"Iceland and Norway in the north are gateways to the Arctic. Turkey in the south borders Syria and Iraq," he said. 

"And in the west, the United States, Canada and the UK link together both sides of the Atlantic."

The European Commission, the bloc's executive, wants to take a stronger geopolitical role, adopting a more assertive foreign policy ad developing European defence industries.

Some EU leaders, notably France's President Emmanuel Macron has gone further, questioning whether NATO's strategy is in a "state of brain death" in the post Cold War world.

But others, including Stoltenberg, see an opportunity in the election of US President Joe Biden to renew trans-Atlantic ties that frayed under Donald Trump's administration.