"The applications you have made today are an historic step. Allies will now consider the next steps on your path to NATO," NATO chief Jens Stoltenberg said, after receiving the bids from the Finnish and Swedish ambassadors at the alliance's headquarters.

The membership push could represent the most significant expansion of NATO in decades, doubling its border with Russia, and President Vladimir Putin has warned it may trigger a response from Moscow. 

But the applications face resistance from NATO member Turkey, which has threatened to block them over accusations the Nordic neighbours act as safe havens for armed groups opposed to Ankara. 

"The security interests of all allies have to be taken into account and we are determined to work through all issues and reach rapid conclusions," Stoltenberg said. 

"All allies agree on the importance of NATO enlargement. We all agree that we must stand together and we all agree that this is an historic moment which we must seize."

Helsinki, Stockholm and the other Western allies remain optimistic they can overcome Turkey's objections. 

NATO ambassadors are expected to discuss the applications on Wednesday and could give the green light on opening formal talks with the pair on their bids. 

Several NATO allies, most notably Britain, have offered security assurances to Finland and Sweden during the application period before they are covered by alliance's mutual defence pact. 

"Over the past few days we have seen numerous statements by allies committing to Finland and Sweden's security. NATO is already vigilant in the Baltic Sea region and NATO and allies forces will continue to adapt as necessary," Stoltenberg said.