Finland and Sweden have officially applied to join the world's biggest military alliance amid a "critical moment" in global security. 

Nato secretary-general Jens Stoltenberg "warmly" welcomed the requests after receiving application letters from the two countries’ ambassadors amid Russia's ongoing invasion of Ukraine.

“This is a good day at a critical moment for our security," he said. 

Their applications will now be considered by the 30 member countries in a process expected to take around two weeks. 

If objections from the likes of Turkish President Recep Tayyip Erdogan are overcome and talks go as expected, the two Nordic countries could become members within a few months despite the process usually taking up to 12 months. 

Both countries have a long history of neutrality and non-alignment but public support for Nato accession has risen since Putin's invasion of Ukraine on February 24.

Mr Stoltenberg added: "The applications you have made today are a historic step.

"Over the past few days, we have seen numerous statements by Allies committing to Finland’s and Sweden’s security.

"NATO is already vigilant in the Baltic Sea region, and NATO and Allies’ forces will continue to adapt as necessary."

The secretary-general said that Nato allies “are determined to work through all issues and reach rapid conclusions”.

“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,” he told reporters at Nato headquarters in Brussels.

The fact that the Nordic partners applied together means they will not be losing time by having to ratify each other’s membership application.

“That Sweden and Finland go hand-in-hand is a strength. Now the process of joining the talks continues,” Swedish foreign minister Ann Linde told the Swedish news agency TT.

Some Nato members have already endorsed the new additions, with Canada announcing intentions to ratify their accession protocol in just a few days.

The prime ministers of the Baltic states of Estonia, Latvia and Lithuania said they "fully endorse" the application in a joint statement on Wednesday.

READ MORE: Finland confirms application for Nato membership