Finland and Sweden are expected to announce their intention to join the alliance on the same day in mid-May. Both Helsinki and Stockholm, despite previously avoiding joining, now support being NATO members after Russia’s invasion of Ukraine.
However, Stockholm is particularly concerned it will be vulnerable to retaliation from Russia for several months before it officially becomes a NATO member.
The decision to admit either country will have to be ratified by all 30 of NATO’s existing member states.
In some cases, members are only approved through parliamentary votes, and the process may last well into the second half of this year.
In response the UK, the US and other NATO members are looking at ways of reassuring Sweden and Finland.
Royal Navy vessels, accompanied by warships from Sweden and four other allies in the region, have carried out “freedom of navigation” exercises in the Baltic over the past few weeks.
Poseidon P8A submarine hunter aircraft from the RAF have also joined the patrols.
Swedish officials believe their country could be treated as a de facto NATO member while their application is approved.
In practice, this would mean “security-promoting measures” such as additional British naval patrols in the Baltic and enhanced intelligence-sharing.
READ MORE: Russia sees NATO blow: Sweden and Finland agree to join ‘within weeks’
A source told Aftonbladet, a Swedish newspaper, NATO will bolster Stockholm’s security while their membership applications are under way.
They said: “It’s about strong political support from Nato countries, in-depth exercises, increased Nato presence in the Baltic region, deeper intelligence co-operation and expert support to counter hybrid, cyber and conventional threats.”
Jens Stoltenberg, NATO’s secretary-general, who is from Norway, has said both Sweden and Finland would be welcomed into the alliance if they chose to join.
Earlier this month, he promised to find ways of “addressing the concerns they may have about this interim period between having applied and until the last ratification [by the individual member states] has taken place”.
Moscow has previously threatened that NATO accession for Sweden and Finland would bring “military and political consequences” for the two countries.
It also said it could deploy nuclear weapons to Kaliningrad, its heavily fortified outpost on the Baltic coast between Poland and Lithuania, in response to such a move.
On Tuesday, Russia expelled three Swedish diplomats in response to what it perceives as unfriendly acts by Stockholm.
The foreign ministry in Moscow said that the expulsions were being made because Sweden had “groundlessly” expelled three Russian diplomats.
Russian Foreign Minister Sergei Lavrov also said on Tuesday Moscow is effectively at war with NATO, and warned that Western weapons are inflaming the conflict in Ukraine.
Mr Lavrov said continual deliveries of supplies and weaponry to Ukraine means the NATO alliance has positioned itself as “in essence engaged in war with Russia” and accused the organisation of “pouring oil on the fire”.
The diplomat also warned against provoking a third world war and said the threat of a nuclear conflict “should not be underestimated”.