Finland and Sweden will be able to join NATO quickly should they decide to ask for membership in the Western military alliance, NATO Secretary-General Jens Stoltenberg said on Thursday.

"If they decide to apply, Finland and Sweden will be warmly welcomed and I expect the process to go quickly," Stoltenberg told reporters in Brussels, adding he planned to speak with the Finnish president later in the day.

According to Reuters, he said he was sure arrangements could be found for the interim period between an application by the two Scandinavian countries and the formal ratification in the parliaments of all 30 NATO members.

"I am confident that there are ways to bridge that interim period in a way which is good enough and works for both Finland and Sweden," Stoltenberg said.

Russia, with which Finland shares a 1,300-km (border, has said it will deploy nuclear weapons and hypersonic missiles in its Baltic enclave of Kaliningrad if Finland and Sweden decide to join NATO.

Finland and Sweden must prepare for increased Russian spy operations, cyber attacks and attempts to influence lawmakers as they consider joining NATO after Moscow's invasion of Ukraine, the Nordic nations' intelligence chiefs said on Wednesday.

Russia's invasion, which it calls a "special operation", has forced Sweden and Finland to reassess their longstanding military neutrality, and they are expected to announce in May whether they will join the alliance.