NATO must become more European to reduce its long-standing reliance on the US security umbrella, Ursula von der Leyen and Mark Rutte said on Tuesday as leaders of the 77-year-old alliance gathered in Ankara, Turkey, for their annual summit.

"We both know how important close cooperation between the European Union and NATO is," the European Commission president said alongside the NATO secretary general at an industrial forum ahead of the summit.

"But to make this possible, what we need is interoperability."

Rutte echoed von der Leyen's remarks, highlighting the "clear division of labour" between the two organisations: NATO oversees the command structure, capabilities and standards, while the EU is responsible for industry, investment and regulation.

Twenty-three of the EU's 27 member states are also members of NATO.