"We are facing a historic moment for NATO," the Spanish prime minister said, expressing hope the Madrid summit would "guarantee and reinforce unity and cohesion" within the alliance.

At the June summit, one of the main aims will be to revise and update NATO's strategy that was last outlined in 2010 in order to prepare for "the (security) challenges facing us", Sanchez said.

The announcement comes just weeks after the announcement of AUKUS, a new military partnership between the US, the UK and Australia which angered France and left some Europeans feeling NATO had been downgraded in Washington's priorities.

"We need to keep adapting for the future and that's exactly what we are going to do at the Madrid summit," Stoltenberg said.

"We see rising strategic competition, more aggressive Russian behaviour and China flexing its economic and military might to intimidate others," he added, also referring to "instability and threats in the Middle East, North Africa and the Sahel".

At the last NATO summit in Brussels in June, the alliance's leaders had set out their "red lines" vis-a-vis Russia and decided to present a united face in respect to China's efforts to enter Europe.

The AUKUS partnership angered France because it torpedoed its submarine mega-contract with the Australians, prompting Paris to voice support for the idea of a European defence group outside of NATO.

But earlier this week, Stoltenberg said the idea of a European-only mutual defence organisation risked dividing and weakening NATO.

"What I don't believe in is any efforts to try to do something outside the NATO framework, or compete with or duplicate NATO, because NATO remains the cornerstone, the bedrock for European security and also actually of North American security," he said on Tuesday.