"The most important thing is that both Finland and Sweden become full members of NATO quickly, not whether they join at exactly the same time," Stoltenberg said.

The head of the Western military alliance said Helsinki's membership "will strengthen Finland's security, it will strengthen Sweden's security, and it will strengthen NATO's security".

He called on the Turkish parliament to vote on the ratification soon.

Turkey and Hungary are the only NATO members still to ratify the Finnish and Swedish applications, which must be accepted by all 30 existing members of the alliance.

Shortly after Turkish President Recep Tayyip Erdogan had announced his decision, Hungary said that its parliament will vote on Finland's membership bid on March 27.

The Nordic neighbours dropped decades-long policies of military non-alignment and applied to join the western alliance last May in the wake of Moscow's invasion of Ukraine.

They initially hoped to join together after their their applications were accepted at a June NATO summit.

But Erdogan has accused the Nordic neighbours of breaking the terms of a deal under which Turkey agreed to approve the bids.

The Turkish leader voiced particular displeasure with Sweden -- a country with a larger Kurdish diaspora and a longer history of disputes with Ankara.

Erdogan's demands became more urgent as he neared a May election in which he will need a strong turnout from his nationalist supporters to extend his two-decade rule.

Stoltenberg has sought to mediate a compromise with Ankara, but so far there has not been any breakthrough on Stockholm's application.

The NATO leader insisted that Finland and Sweden's membership had already been bolstered by their push to join.

"It is inconceivable that NATO would not respond should either Finland or Sweden come under attack. Their security matters to NATO," he said.