US President Joe Biden on Thursday welcomed the leaders of Finland and Sweden in a strong show of support for their bids to join NATO in the face of Russia's invasion of Ukraine.

To the pomp of a red carpet and military honor guard, Biden received Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinisto just days after they officially said they would seek to join the US-backed alliance.

The three leaders are expected to speak to reporters after talks in the Oval Office.

Sweden and Finland, while solidly Western, have historically kept a distance from NATO as part of longstanding policies aimed at avoiding angering Russia.

But the two nations both moved ahead amid shock over their giant neighbor's invasion of Ukraine, which had unsuccessfully sought to join NATO.

Biden on Wednesday said he "strongly" supported the membership of Sweden and Finland in the alliance, which considers an attack on one member an attack on all.

"While their applications for NATO membership are being considered, 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 the threat of aggression," he said in a statement.

In the United States it is up to the Senate to ratify treaties and there is wide support for the membership of Sweden and Finland, with votes likely before a NATO summit next month in Madrid.

But Turkey has voiced misgivings about the membership of the two countries, accusing them of supporting "terrorism" due to the presence of Kurdish militants.

All 30 current members of NATO would need to agree to the Swedish and Finnish bids.