WASHINGTON - President Joe Biden on Monday thanked Greek Prime Minister Kyriakos Mitsotakis for his country's 'moral leadership' in the aftermath of Russia's invasion of Ukraine as the two held talks at the White House about the ongoing conflict.

The visit by Mitsotakis comes as he was in Washington to mark a COVID-delayed commemoration of the bicentennial of the start of the Greek War of Independence, a more than eight-year-long struggle that led to the ouster of the Ottoman Empire. The president and first lady Jill Biden hosted Mitsotakis and his wife, Mareva Grabowski-Mitsotakis, later Monday at a White House reception to mark the bicentennial.

But the celebratory moment was overshadowed by the most significant fighting on the continent since World War II, and as Biden seeks to keep the West unified as it pressures Russia to end the war.

Mareva Grabowski, wife of Greek Prime Minister Kyriakos Mitsotakis, and first lady Jill Biden applaud during a reception with President Joe Biden in the East Room of the White House in Washington, May 16, 2022.

'We are now facing united the challenge of Russian aggression,' Mitsotakis said at the start of his meeting with Biden. The prime minister added that the U.S.-Greek relationship was at an 'all-time high.'

As Europe looks to wean itself off Russian energy, Mitsotakis has pushed the idea of Greece becoming an energy hub that can bring gas from southwest Asia and the Middle East to Eastern Europe.

A new Greece-to-Bulgaria pipeline - built during the COVID-19 pandemic, tested and due to start commercial operation in June - is slated to bring large volumes of gas between the two countries in both directions to generate electricity, fuel industry and heat homes.

The new pipeline connection, called the Gas Interconnector Greece-Bulgaria, will give Bulgaria access to ports in neighboring Greece that are importing liquefied natural gas, or LNG, and also will bring gas from Azerbaijan through a new pipeline system that ends in Italy. Russia announced last month it was cutting off natural gas exports to Bulgaria and Poland over the countries' refusal to pay in rubles.

The Oval Office meeting with Biden also comes after Greece, a fellow NATO nation, last week formally extended its bilateral military agreement with the United States for five years, replacing an annual review of the deal that grants the U.S. military access to three bases in mainland Greece as well as the American naval presence on the island of Crete.

Mitsotakis has expressed support for Finland and Sweden seeking membership in the NATO defense alliance, a development welcomed by much of the 30-nation group with the notable exception of Tukey, which remains locked in a decades-old dispute with Greece on sea boundaries and mineral rights in the eastern Mediterranean.

FILE - Turkish President Tayyip Erdogan speaks during a news conference in Kyiv, Ukraine, Feb. 3, 2022.

Turkish President Recep Tayyip Erdogan on Monday again voiced some objections to accepting Finland and Sweden, accusing the two countries of supporting Kurdish militants and others whom Turkey considers to be terrorists.

'Neither country has an open, clear stance against terrorist organizations,' Erdogan said at a joint news conference with the visiting Algerian president. 'We cannot say 'yes' to those who impose sanctions on Turkey, on joining NATO, which is a security organization.'

Mitsotakis, in an appearance on MSNBC's 'Morning Joe' on Monday, expressed optimism that Turkey, in the end, won't hold up Finland and Sweden's bid to join NATO and addressed speculation that Erdogan might use the moment to win concessions from the Biden administration on weapons sales or other matters.

'This is not really the right time to use a NATO membership (application) by these two countries to bargain' for other issues,' he said.

In addition to his address to Congress, Mitsotakis is scheduled Tuesday to be honored at a luncheon hosted by House Speaker Nancy Pelosi and will meet with members of the Congressional Caucus on Hellenic Issues and members of the Senate Foreign Relations Committee.