Sweden and Finland have the "full, total and complete backing" of the US in their decision to apply for Nato membership, President Joe Biden says.
Both countries submitted their applications to be part of the Western defence alliance this week, marking a major shift in European geopolitics.
To join the alliance, the two nations need the support of all 30 Nato member states.
But the move by the Nordic nations has been opposed by Turkey.
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 "a watershed moment in European security".
"New members joining Nato is not a threat to any nation," he said. The president added that having two new members in the "high north" would "enhance the security of our allies and deepen our security co-operation across the board".
Russia has repeatedly said it sees Nato as a threat and has warned of "consequences" if the block proceeds with its expansion plans.
Turkey has accused both Sweden and Finland of hosting suspected militants from the Kurdistan Workers' Party (PKK), a group it views as a terrorist organisation.
However, both Nato Secretary-General Jens Stoltenberg and British Defence Minister Ben Wallace have expressed confidence that these concerns will eventually be addressed.
Mr Biden's comments came as the US Senate voted to approve a new $40bn (£32bn) bill to provide military and humanitarian aid to Ukraine. It is the biggest emergency aid package so far for Ukraine.
The bill - which was passed by the House of Representatives with broad bipartisan support on 10 May - was expected to be passed earlier this week, but was blocked by Kentucky Republican Rand Paul over a dispute about spending oversight.
But the Republican's Senate leader Mitch McConnell dismissed these concerns and told reporters that Congress had a "moral responsibility" to support "a sovereign democracy's self-defence".
"Anyone concerned about the cost of supporting a Ukrainian victory should consider the much larger cost should Ukraine lose," Mr McConnell said.
Last week, Defence Secretary Lloyd Austin and Secretary of State Antony Blinken urged Congress to approve the package and warned that the US military only had enough funds to send weapons to Kyiv until 19 May.
Ukraine's President Volodymyr Zelensky praised the aid package as "a significant US contribution to the restoration of peace and security in Ukraine, Europe and the world".
The package brings the total US aid delivered to Ukraine to more than $50bn, including $6bn for security assistance such as training, equipment, weapons and support.
Another $8.7bn will be allocated to replenish stocks of US equipment already sent to Ukraine.