Prospective NATO member Finland committed on Thursday to ensuring Turkey's security if its bid to join the trans-Atlantic alliance is successful, as U.S. President Joe Biden hailed the "momentous" applications of once-neutral Sweden and Finland to join the bloc in response to Russia's invasion of Ukraine while the three countries worked to address continued opposition from Turkey.

Finnish President Sauli Niinisto acknowledged Ankara's concerns about his nation's membership bid but maintained that joining NATO would ensure both nations commit to their mutual security.

"Finland has always had proud and good bilateral relations with Turkey. As NATO allies, we will commit to Turkey's security, just as Turkey will commit to our security," Niinisto said at the White House where he was hosted by Biden alongside Swedish Prime Minister Magdalena Andersson.

"We take terrorism seriously. We condemn terrorism in all its forms and we are actively engaged in combating it. We are open to discussing all the concerns Turkey may have concerning our membership in an open and constructive manner," he added.

For her part, Andersson said her government is "right now having a dialogue with all NATO member countries, including Turkey, on different levels to sort out any issues at hand."

Biden greeted Prime Minister Andersson of Sweden and President Niinisto of Finland at the White House with handshakes and laughter as they met for trilateral conversations on the NATO mutual defense pact as well as broader European security concerns. His administration has professed optimism for their applications to join the alliance, which would mark a significant embarrassment to Russia after its invasion of Ukraine.

"Today I’m proud to welcome and offer the strong support in the United States for the applications of two great democracies, and two close, highly capable partners to join the strongest, most powerful defensive alliance in the history of the world," Biden said after escorting his fellow leaders to the Rose Garden.

"They meet every NATO requirement and then some," he said, and "having two new NATO members in the high north will enhance the security of our alliance."

Russian President Vladimir Putin has demanded that the alliance stop expanding toward Russia’s borders, and several NATO allies, led by the United States and Britain, have signaled that they stand ready to provide security support to Finland and Sweden should the Kremlin try to provoke or destabilize them during the time it takes to become full members. Putin cited Ukraine's aspirations to join NATO as one reason for his invasion of the country.

"New members joining NATO is not a threat to any nation," Biden said in implicit rejection of Putin. "It never has been."

The leaders' optimism for Sweden and Finland's applications was set against lingering opposition from Turkey, whose President Recep Tayyip Erdoğan said in a Thursday video that he remains opposed. Each of NATO's 30 member countries has the power to veto a membership bid.

"We have told our relevant friends we would say ‘no’ to Finland and Sweden’s entry into NATO, and we will continue on our path like this," Erdoğan told a group of Turkish youth in the video for Commemoration of Atatürk, Youth and Sports Day, a national holiday.

Erdoğan has said Turkey’s objection stems from grievances with Sweden’s, and to a lesser degree with Finland’s, perceived support of the banned terror group PKK and its Syrian branch YPG.

Turkey also accuses Sweden and Finland of harboring the followers of the Gülenist Terror Group (FETÖ), which was behind the 2016 military coup attempt.

The objections echo longtime Turkish complaints about even more substantial U.S. support for these terrorist groups.

Erdoğan branded the two prospective NATO members and especially Sweden as "a focus of terror, home to terror." He accused them of giving financial and weapons support to the armed groups, and claimed the countries' alleged links to terror organizations meant they should not be part of the trans-Atlantic alliance.

Turkish officials also have pointed to arms restrictions on Turkey as a reason for Ankara’s opposition to the two countries becoming part of NATO.

Several European countries, including Sweden and Finland, restricted arms exports to Turkey following the country’s cross-border operation into northeast Syria in 2019 with the stated goal of clearing the border area of terror groups.

"I think we’re going to be okay," Biden said Wednesday when asked whether he was confident he could secure their entry into NATO.

While neutral throughout the Cold War, Finland and Sweden now cooperate closely with NATO. The countries will only benefit from NATO’s Article 5 security guarantee, the part of the alliance’s founding treaty that pledges that any attack on one member would be considered an attack on them all, once the membership ratification process is concluded. Public opinion in Finland and Sweden has shifted massively in favor of membership since Russia invaded Ukraine on Feb. 24.

"After 200 years of military nonalignment Sweden has chosen a new path," Andersson said, calling Russia’s invasion a "watershed moment" causing her country to rethink its security posture. "My government has come to the conclusion that the security of the Swedish people will be best protected within the NATO alliance," she said.

Biden on Thursday announced that he was asking the U.S. Senate to approve the new NATO memberships, pending Sweden and Finland's completion of the ratification process. The required step was expected to receive overwhelming, bipartisan support.

U.S. National Security Adviser Jake Sullivan said later in the day that Turkey's concerns regarding Sweden and Finland's entry into NATO can be resolved.

"We believe that the Turkish concerns about the accession of Sweden and Finland, they've been expressed by President Erdoğan and others, can be addressed and can be resolved," Sullivan told reporters as Biden was departing for South Korea and Japan.

As Biden hosted Andersson and Niinistö at the White House earlier in the day, Sullivan said "they're both intending to speak directly" to Erdoğan and his team.

"We've indicated if there's anything we can do to be supportive, we will do it, but fundamentally this is an issue about concerns that Turkey has raised," said Sullivan, adding those would be on Finland and Sweden.

The White House also indicated that it does not expect it will need to revisit the F-16 deal with Turkey as part of negotiations over Nordic countries' NATO bid.

Turkey was dropped from the F-35 advanced fighter jet development program after it bought a Russian air defense system. Since then, Turkey has been pressing the U.S. to sell it new F-16 fighters or at the very least to refurbish its existing fleet.

NATO is in close contact with Finland, Sweden and Turkey, the alliance's chief also said on Thursday.

"We are addressing the concerns that Turkey has expressed," Jens Stoltenberg told a joint press conference with Denmark's Prime Minister Mette Frederiksen in Copenhagen.

"Because when an ally, an important ally as Turkey, raises security concerns, raised these issues, then, of course, the only way to do that is to sit down and find ways to find a common ground and an agreement on how to move forward," he added.

NATO's enlargement has been a great success, Stoltenberg said, adding that "every country has the right to choose its own path, that of course also includes Finland and Sweden."

"We are 30 allies from both sides of the Atlantic with different history, geography, political parties in government, and sometimes there are some differences. But we have a long track record in NATO of being able to overcome differences," the alliance chief said.

A statement also came from the United Kingdom earlier in the day. NATO will "certainly be listening" to Turkey's concerns over Sweden and Finland's membership bids, the British defense secretary said on Thursday.

"Turkey is an incredibly important member of NATO and indeed, a strong contributor," said Ben Wallace in his address to the House of Commons.

"And we should always remember that NATO covers a very wide frontier from the high north, the Arctic in Norway, all the way through to the Black Sea, and to Turkey. And Turkey is very important, one of the oldest members of NATO," he said, adding "that we understand in this environment, what Turkey is concerned about and addressed them in order to make sure that the 30 nations come together to support and accept Finland and Sweden."

Wallace said he will be speaking to his Turkish counterpart Hulusi Akar, and that he heard President Erdoğan's speeches on being "worried about PKK terrorism groups and whether members are doing enough to deal with it."

"I think there is a way through. I think we will get there in the end, and it is very important that we listen to all members and their concerns in that process and we will certainly be listening to Turkey."

Finland and Sweden formally applied to join NATO on Wednesday, a decision spurred by Russia's war on Ukraine that began in February. But Turkey, a longstanding member of the alliance, has voiced objections to the membership bids, criticizing the countries for tolerating and even supporting terrorist groups.

Over the last five years, both Helsinki and Stockholm have failed to agree to Ankara's requests for the extradition of dozens of terrorists, including members of the PKK, YPG and FETÖ.

Senior representatives of Finland and Sweden are set to visit Ankara to discuss their accession process, which needs the unanimous approval of all 30 members.