Britain has pledged to come to Sweden and Finland's aid if the countries came under attack.

British prime minister Boris Johnson and Swedish prime minister Magdalena Andersson signed a security assurance deal outside Stockholm on Thursday.

Sweden and Finland are pondering whether to join NATO following Russia's invasion of Ukraine on February 24.

Johnson later made a similar defence commitment to Finland, where he met with President Sauli Niinisto.

The agreement with Sweden will “fortify northern Europe’s defences, in the face of renewed threats,” Johnson said in a statement, adding it “is a symbol of the everlasting assurance between our nations."

“These are not a short-term stop-gap, but a long-term commitment to bolster military ties and global stability, and fortify Europe’s defences for generations to come,” Johnson said in the statement. Finland shares a 1,340-kilometre land border with Russia.

“And whether it’s in the event of a disaster or a military attack, what we’re saying today is that upon request from the other party, we would come to the other parties’ assistance,” Johnson told a joint news conference. He said that the war in Ukraine is Russian President Vladimir “Putin’s bloodthirsty campaign against a sovereign nation.”

Andersson said: “Putin thought he could cause division, but he has achieved the opposite. We stand here today more united than ever."

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The Kremlin has warned of “military and political repercussions” if Sweden and Finland decide to join NATO. Andersson said that Russia would increase its “military presence in this region if Sweden and, or, Finland sends in an application.”

Should they apply, there will be an interim period lasting from when an application has been handed in until all 30 NATO members’ parliaments have ratified it. The two Nordic countries are expected to announce their positions on NATO membership in the coming days.

“If Finland makes this historical step it is for the security of our own citizens,” Finnish Prime Minister Sanna Marin told a news conference after talks with Japanese Prime Minister Fumio Kishida in Tokyo overnight: “Joining NATO will strengthen the whole international community that stands for common values.”

Johnson met with Andersson in Harpsund, the country retreat of Swedish prime ministers, which is located about 90 kilometres southwest of Stockholm. He met with with Niinisto, who has a significant role in Finland’s foreign and security policy decisions, at the Presidential Palace in Helsinki on Thursday.

Johnson, who said Putin was “a 21st-century tyrant,” also offered during his one-day visit to increase the deployments of British troops and military assets to the region.

“And in times of crisis, cooperation becomes even more important. And this applies not least for our international defence partnerships. And Sweden’s partnerships with the UK and with NATO have been crucial during these exceptional times,” Andersson said.

Britain is already present in the Baltic Sea areas with the Joint Expeditionary Force, which consists of 10 Northern European nations: the United Kingdom, Sweden, Finland, Denmark, Estonia, Iceland, Latvia, Lithuania, the Netherlands and Norway.

In 2017, Sweden and Finland joined the British-led military rapid reaction force, which is designed to be more flexible and respond more quickly than the larger NATO alliance. It uses NATO standards and doctrine, so it can operate in conjunction with NATO, UN or other multinational coalitions. Fully operational since 2018, the force has held a number of exercises both independently and in cooperation with NATO.

Mari Yamaguchi in Tokyo, Japan contributed to this report.