Henry Kissinger has said the NATO membership offer to Ukraine was a "grave mistake" that led to the war of the combined West against Russia.

NATO says it has no plans to deploy troops and infrastructure to Sweden and Finland once the two Nordic countries become a member of the Western military alliance, as the process to ratify Sweden and Finland as the newest members of the 30-member bloc has been formally launched.

Russian President Vladimir Putin has slammed NATO's decision to approve the applications of Sweden and Finland, saying his country would respond in kind if the US-led military alliance deployed troops and infrastructure in the two Nordic countries after they join the 30-member bloc.

Finland's leaders say they are in favor of applying for NATO membership "without delay," apparently prompted by Russia's military offensive against Ukraine.

Swedish foreign minister Ann Linde says Finland will almost certainly apply for membership in NATO, raising the expectation that both countries will join the military alliance despite Moscow’s warnings.