Britain has pledged to come to the aid of Sweden and Finland, including with military support, should the two Nordic nations come under attack.
UK Prime Minister Boris Johnson signed security declarations with his Swedish counterpart in Stockholm and the Finnish President in Helsinki on Wednesday.
Despite a long history of neutrality, the two countries are considering joining NATO following Russia's invasion of Ukraine.
The new agreements would "fortify northern Europe's defences, in the face of renewed threats," Mr Johnson said, calling them symbols "of the everlasting assurance between our nations".
Wednesday's agreements also cover closer collaboration on new technology and intelligence-gathering between Britain and the two Nordic countries.
Mr Johnson said he would offer to increase British military deployments to the region, including with air force, army and navy assets and personnel.
Earlier this year he had said the deals would allow Britain, a major, nuclear-armed NATO member, "to cooperate with key Nordic partners and their armed forces, in all domains, including cyberspace".
Sweden's eastern neighbour Finland shares a 1,340-kilometre land border with Russia.
'We stand here today more united than ever'
"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," Mr Johnson said.
"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 party's assistance," he later told a joint news conference in Sweden with Prime Minister Magdalena Andersson.
He called the war in Ukraine Russian President Vladimir Putin's "bloodthirsty campaign against a sovereign nation".
Ms Andersson added: "Putin thought he could cause division, but he has achieved the opposite. We stand here today more united than ever."
In Finland, Mr Johnson held talks with President Sauli Niinisto, who has a significant role in the nation's foreign and security policy decisions.
The Finnish head of state said Moscow could only blame itself should his nation of 5.5 million become a NATO member.
"You [Russia] caused this. Look at the mirror," Mr Niinisto said pointedly, "So in my thinking, this is quite simple, actually. We increase our security and we do not take it away from anybody. It is not a zero-sum game."
The Kremlin has warned of "military and political repercussions" if Sweden and Finland decide to join NATO.
"If Finland makes this historical step it is for the security of our own citizens," Finnish Prime Minister Sanna Marin said during a visit to Japan.
"Joining NATO will strengthen the whole international community that stands for common values."
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.