Biden saw Niinisto briefly during his trip to Washington, where he was meeting with US senators and White House National Security Advisor Jake Sullivan, after earlier trips this week to Seattle and California.

Biden dropped in on the Sullivan meeting and "reiterated our strong support for Finland's swift accession to NATO," Principal Deputy Press Secretary Olivia Dalton told reporters.

The White House said Biden and Niinisto also "discussed their continued commitment to supporting Ukraine in the face of Russia's aggression."

Along with Sweden, Finland has applied to join NATO in response to the more than year-long war unleashed by Russia against Ukraine.

The two countries' accession requires unanimous consent from the 30 current members. Turkey and Hungary have yet to give the green light.