President Joe Biden has said Sweden and Finland have the "full, total, complete backing" of the US in their historic decision to apply for Nato membership.
Both countries submitted their applications this week to be part of the defence alliance, a drastic shift in European geopolitics.
The move by the two Nordic nations has been opposed by Nato ally Turkey.
Russia sees Nato as a threat and has warned of "consequences" to expansion.
To join the alliance, the two nations need the support of all 30 Nato member states.
Speaking alongside Swedish Prime Minister Magdalena Andersson and Finnish Prime Minister Sauli Niinisto at the White House on Thursday, Mr Biden called Sweden and Finland's applications to join Nato "a watershed moment in European security".
Mr Biden added that having two new members in the "high north" will "enhance the security of our allies and deepen our security cooperation across the board".