US President Joe Biden has signed documents backing the membership of Sweden and Finland in the NATO military alliance.
"[Russian President Vladimir] Putin thought he could break us apart," Biden said on Tuesday.
"Our alliance is closer than ever, it is more united than ever and after Finland and Sweden join we will be stronger than ever," he added.
"President Biden congratulated them on the US Senate's swift, bipartisan ratification of their NATO accession protocols, and welcomed Finland and Sweden moving one step closer to becoming NATO Allies," the White House said.
While the process plays out, "President Biden affirmed that the United States will work with Finland and Sweden to remain vigilant against any threats to our shared security, and to deter and confront aggression or threat of aggression," the statement said.
This is the most significant expansion of the North Atlantic Alliance since the 1990s.
The US Senate backed the expansion by an overwhelming 95-1 last week.
Both Democratic and Republican Senators strongly approved membership for the two European countries, describing them as important allies whose modern military forces were already working closely with NATO.
Sweden and Finland bid for NATO membership after Russia launched a special military operation in Ukraine in late February.
Moscow has repeatedly warned against NATO's expansion, describing it as a US-led effort by the West to annihilate Russia.
Russia’s deputy foreign minister Sergei Ryabkov said earlier this year that Moscow views NATO's expansion in Europe as a threat and destabilizing factor.
"We consider the expansion of the North Atlantic alliance to be a purely destabilizing factor in international affairs. It does not add security either to those who are expanding it, those joining it or to other countries that perceive the alliance as a threat," the Interfax news agency quoted Ryabkov as saying.
Moscow has repeatedly warned the West against the alliance’s expansion towards its borders, vowing to take appropriate measures against the threat posed by NATO to the Russian motherland.