President Joe Biden said Thursday that the United States supports the bids of Sweden and Finland to join NATO.
The two countries would make NATO stronger and Biden called their move to join the pact a victory for democracy.
The president said he would work with Congress to approve NATO bids.
Biden said following a trilateral meeting with the leaders that NATO is needed more than ever.
Russia's assault on Ukraine has raised fears for other countries in the region. Russia has opposed the two nations' plans to join the alliance.
Russian President Vladimir Putin said earlier in the week that the entry of Sweden and Finland into NATO will cause a response.
The Russian Foreign Ministry said in a statement on Monday that Russia will be forced to take retaliatory steps, both of a military-technical and other nature, in order to stop the threats to its national security arising in this regard.
Many of the requirements to be a NATO member are already met by both Sweden and Finland. Having a functioning democratic political system, willingness to provide economic transparency, and the ability to make military contributions to NATO are some of the things that are included.
NATO members must approve a country's bid for membership in the alliance.
Turkey's president said he wouldn't approve NATO applications from Sweden and Finland. He cited their support for Kurdish organizations that Turkey considers security threats.
He said that the nations shouldn't bother coming to Turkey to try to convince him.
During remarks in the White House Rose Garden, Niinisto and Andersson addressed Erdogan's concerns.
Niinisto said that they take terrorism seriously and condemn terrorism in all its forms.
Sweden is talking to all NATO member countries and holding more talks with Turkey to discuss specific concerns.