"Finland must apply for NATO membership without delay. We hope that the national steps still needed to make this decision will be taken rapidly within the next few days," said Niinistö and Marin in a joint statement.
Kremlin spokesman Dmitry Peskov said that Russia's response to the move would depend on what specific steps NATO will take to bring its infrastructure close to Russian borders. He noted that Russian President Vladimir Putin already had ordered to work out steps to strengthen the country's defences in the west in response to NATO's expansion closer to Russian territory.
In NATO member Estonia, which also borders Russia, Prime Minister Kaja Kallas tweeted that "history being made by our northern neighbours." She pledged to support "a rapid accession process" for Finland into NATO.
Finland's announcement came a day after British Prime Minister Boris Johnson visited both Finland and Sweden to sign a military cooperation agreement.
Since Russia's invasion of Ukraine on Feb. 24, Finland and Sweden have been assessing whether to abandon their decades-old neutrality and join the 30-member NATO. After Moscow launched its attack on Ukraine, public support in the two countries started to quickly shift toward membership in NATO, first in Finland and a bit later in Sweden.
The latest opinion poll conducted by Finnish public broadcaster YLE showed earlier this week that 76% of Finns are in favour of joining NATO, a big change from earlier years when only 20-30% of respondents favored such military alignment.
Swedish Foreign Minister Ann Linde tweeted that Finland's announcement gave an "important message" and Danish Prime Minister Mette Frederiksen said that there were "strong messages" from Finland's president and prime minister.
During the Cold War, Finland stayed away from NATO to avoid provoking the Soviet Union, instead opting to remain a neutral buffer between the East and the West while maintaining good relations with Moscow and also with the United States.
