U.S. President Joe Biden on Tuesday signed documents endorsing Finland and Sweden's accession to NATO, the most significant expansion of the military alliance since the 1990s as it responds to Russia's invasion of Ukraine.

Biden signed the U.S. "instrument of ratification" welcoming the two countries, 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 Two 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 worked closely with NATO.

The vote was a sharp contrast with some rhetoric in Washington during the administration of former Republican President Donald Trump, who pursued an "America First" foreign policy and criticized NATO allies who failed to reach defense spending targets.

Sweden and Finland applied for NATO membership in response to Russia's Feb. 24 invasion of Ukraine. Moscow has repeatedly warned both countries against joining the alliance.

Putin is getting "exactly what he did not want," with the two countries entry into the alliance, Biden said.

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.