U.S. President Joe Biden signed the Protocols to the North Atlantic Treaty of 1949 on the Accession of the Republic of Finland and the Kingdom of Sweden on Tuesday amid the Russia-Ukraine conflict, according to the White House.

This is the final step for their endorsement by the United States.

"It was and is a watershed moment I believe in the alliance and for the greater security and stability not only of Europe and the United States but of the world," he said of their entry into the post World War II alliance.

The U.S. Senate backed the expansion by an overwhelming 95-1 last week, a rare display of bipartisan unity in a bitterly divided Washington.

Both Democratic and Republican Senators strongly approved membership for the two Nordic countries, describing them as important allies whose modern militaries already work closely with NATO.

NATO's 30 allies signed the accession protocol for Sweden and Finland last month, allowing them to join the nuclear-armed alliance once all member states ratify the decision.

The accession must be ratified by the parliaments of all 30 North Atlantic Treaty Organization members before Finland and Sweden can be protected by Article Five, the defense clause stating that an attack on one ally is an attack on all.

Ratification could take up to a year, although the accession has already been approved by a few countries including Canada, Germany and Italy.

(With input from Reuters)