Finland's president and prime minister announced their support for joining NATO on Thursday, moving the Nordic nation which shares an 800-mile border with Russia one step closer to membership of the US-led military alliance. The Kremlin has responded by saying the move would be a threat to Russia and warned of possible retaliation.

The statement of support for NATO from President Sauli Niinisto and Prime Minister Sanna Marin had been expected, after the Finnish government recently submitted a report on national security to the country's parliament which outlined the path to joining the alliance as one of Finland's options.

In the joint statement, Niinisto and Marin said: "NATO membership would strengthen Finland's security. As a member of NATO, Finland would strengthen the entire defence alliance. 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."

Later on Thursday, Russia's Foreign Ministry said the Finnish statement marked a "radical change in the country's foreign policy" and warned of countermeasures.

"Helsinki must be aware of the responsibility and consequences of such a move," said the ministry.

Finland's possible accession to NATO would cause serious damage to bilateral Russian-Finnish relations, which are maintaining stability and security in the Northern European region, the ministry said.

"Russia will be forced to take retaliatory steps, both of a military-technical and other nature, in order to stop the threats to its national security that arise in this regard," it said.

Also on Thursday, Kremlin spokesperson Dmitry Peskov said that Finland joining the alliance would not contribute to more security.

"As we have said many times before, NATO expansion does not make the world more stable and secure," Peskov told reporters. He added that Russia's reaction would depend on NATO activity near its borders.

"It will depend on what this expansion process will entail, how far and how close to our borders the military infrastructure will move," Peskov said.

The spokesperson added that Russia will analyze the situation with Finland's entry to NATO and will work out the necessary measures to ensure its own security.

Peskov also told reporters on a regular conference call that: "Everyone wants to avoid a direct clash between Russia and NATO: both Russia and NATO, and, most importantly, Washington." He added, however, that Russia will be ready to give "the most decisive response" to those who would try to get involved in the country's "special military operation" in Ukraine -- the Kremlin's official euphemism for Russia's war there.

Since the Russian invasion of Ukraine in February, public support for joining NATO in Finland has leaped from around 30% to nearly 80% in some polls.

The move would require approval by Finland's parliament and the clearance of any other domestic legislative hurdles, but NATO Secretary General Jens Stoltenberg has said that the country would be "warmly welcomed" into the alliance if it applies for membership.

"I welcome the joint statement by President Niinisto and Prime Minister Marin supporting an application for NATO membership without delay," Stoltenberg said Thursday, according to his office.

"This is a sovereign decision by Finland, which NATO fully respects. Should Finland decide to apply, they would be warmly welcomed into NATO, and the accession process would be smooth and swift. Finland is one of NATO's closest partners, a mature democracy, a member of the European Union, and an important contributor to Euro-Atlantic security," he said.

The NATO chief said he agreed with Niinisto and Marin, "that NATO membership would strengthen both NATO and Finland's security."

It is also expected that Sweden, Finland's neighbor to the west, will soon announce its intention to join the alliance through a similar process.

Russia has previously warned both countries against joining NATO, saying there would be consequences.

European diplomats and security officials widely assume that Finland could join the alliance quickly once negotiations start, as it has been buying military hardware compatible with its Western allies, including the US, for decades and already meets many of the criteria for membership.

Finland joining NATO would have both practical and symbolic consequences for Russia and the Western alliance.

Since the end of World War II, Finland has been militarily non-aligned and nominally neutral in order to avoid provoking Russia. It has indulged the Kremlin's security concerns at times and tried to maintain good trading relations.

The war in Ukraine, however, has sufficiently changed the calculation, so that joining NATO now seems the best way forward, regardless of what Russia's reaction might be.

European defense officials who have talked to CNN in recent months assume that NATO countries will offer some guarantees around Finland's security during the accession process, in case Russia retaliates before it has formally joined.

On Wednesday, UK Prime Minister Boris Johnson announced new security pacts with Finland and Sweden, pledging to come to either country's assistance if one of them came under attack.

Finland has historically had high defense spending and still has a policy of conscription, with all adult men liable to be called up for military service. It is widely acknowledged among NATO officials that Finland joining the alliance would be a significant boost in countering Russian aggression because of how seriously the country has historically treated its own security.

It also shares more than 800 miles of border with Russia, which is significant as the Kremlin stated before invading Ukraine that it wanted to see NATO roll back its borders to where they were in the 1990s.

Instead, President Vladimir Putin's gambit may result in a stronger NATO creeping closer.

- by Luke McGee, CNN