Sweden and Finland will finally join NATO as the bloc signed the accession protocol for the countries on Tuesday.

The signing is the most significant expansion of the alliance since the 1990s and it follows a deal with Turkey at last week's NATO summit in Madrid, where Ankara lifted its veto on the Nordic membership bids.  

"This is truly an historic moment," NATO Secretary-General Jens Stoltenberg said alongside the foreign ministers of the two countries. "With 32 nations around the table, we will be even stronger."

He said the process to ratify Sweden and Finland as the newest members of the US-led military alliance has been formally launched.

"We are tremendously grateful for all the strong support that our accession has received from the allies," Swedish Foreign Minister Ann Linde said. "We are convinced that our membership would strengthen NATO and add to the stability in the Euro Atlantic area."

Moscow has repeatedly warned against the alliance’s further expansion toward its borders.

In his remarks in May on the Nordic countries’ bid to join NATO, Russian President Vladimir Putin said the expansion of [the alliance’s] military infrastructure into this territory would "certainly provoke our response."

Citing “security” concerns, Finland and Sweden applied to join NATO after Russia launched a special military operation in Ukraine on February 24.

The military operation was launched following Kiev’s failure to implement the terms of the Minsk agreements and Moscow’s recognition of the breakaway regions of Donetsk and Luhansk.

At the time, Putin said one of the goals of what he called a “special military operation” was to “de-Nazify” Ukraine.

Since the start of Russia’s offensive in Ukraine, the US and its allies have imposed unprecedented sanctions on Moscow.

Russia on Tuesday warned that the implementation of the proposal would lead to significantly less oil on the market and could push prices above $300-$400 a barrel.

Russia's former president Dmitry Medvedev said Japan "would have neither oil nor gas from Russia, as well as no participation in the Sakhalin-2 LNG project" as a result.

G7 leaders agreed last week to explore the feasibility of introducing temporary import price caps on Russian fossil fuels, including oil, in an attempt to limit Russian resources to finance its military campaign in Ukraine.

"There will be significantly less oil on the market, and its price will be much higher. Moreover, higher than the predicted astronomical price of $300-400 per barrel," Medvedev, now deputy chairman of Russia's Security Council, wrote on social media.