Ahead of NATO summit, Biden meets with King Charles for 1st time since coronation

President Joe Biden met with King Charles III for the first time since his coronation on Monday at Windsor Castle.

Biden was greeted with a ceremonial arrival and an inspection of the Honor Guard, with the band playing a rendition of "The Star Spangled Banner" upon his arrival.

There, Biden participated in a climate engagement with King Charles, philanthropists and investors. John Kerry, the president's climate envoy, was also in attendance as was his U.K. counterpart Grant Shapps, the secretary of state for energy security and net zero.

"There's a full understanding here, particularly with the evidence of the last few weeks piling on, that when scientists are telling us they are terrified by what they're seeing, and when we hear those same scientists telling us we are in uncharted territory, this group has come together to try to figure out, 'Ok how do we deploy the funds necessary to invest to create the new clean energy economy,'" Kerry said.

The king has made climate a top issue in his decades of public service.

Biden did not attend the king's coronation in May. First lady Jill Biden and their granddaughter Finnegan Biden represented the White House.

Earlier on Monday, the president met with Prime Minister Rishi Sunak outside 10 Downing Street. The two last met at the White House in June to discuss Ukraine, climate change and other issues.

Biden joked the two had been meeting "once a month" since Sunak became prime minister -- Monday's meeting was the sixth in six months -- and said he "couldn't be meeting with a closer friend, and a greater ally."

"We've got a lot to talk about. I think we're doing well," Biden said. "We're moving along in a way that's positive. But our relationship is rock solid."

London was Biden's first stop ahead of his trip to Vilnius, Lithuania for the 74th NATO summit.

Biden and Sunak reviewed plans for the summit, the White House said, and "reaffirmed their steadfast support for Ukraine in the face of Russia's ongoing aggression."

The summit comes amid a critical point in the Russia-Ukraine war, and is expected to focus on continued aid for Ukraine and the possibility of Sweden joining the alliance.

Sweden formally applied to join the military alliance in May 2022, following in the footsteps of neighboring Finland in ending its long-standing position of neutrality amid the Russia-Ukraine conflict.

Finland officially joined NATO in April but Sweden's bid has been blocked by opposition from Turkey and Hungary. Biden met with Sweden Prime Minister Ulf Kristersson at the White House last week in a show of support for the Nordic nation's bid.

Before departing for his foreign trip, Biden spoke with Turkish President Recep Tayyip Erdogan and "conveyed his desire to welcome Sweden into NATO as soon as possible," according to the White House.

The president did not respond to questions shouted at him Monday about Ukraine or his call with Erdogan during his visit with Sunak.

Ukraine has also expressed desire to join the alliance, though Biden and other officials have said the nation isn't ready to be a part of NATO until the war is over and they implement some political and economic reforms.

Ukrainian President Volodymyr Zelenskyy told ABC Chief Global Affairs Correspondent Martha Raddatz in a wide-ranging interview Sunday for "This Week" that the upcoming NATO summit was "very important" but did not confirm whether or not he will attend.

"If there is no unity regarding the technical invitation for Ukraine to join the alliance, it's all a matter of political will just to find the proper wording and invite Ukraine," Zelenskyy said.

"It would be an important message to say that NATO is not afraid of Russia. Ukraine should get clear security guarantees while it is not in NATO. And that is a very important point. Only under these conditions our meeting would be meaningful."

-ABC's Justin Gomez, Cheyenne Haslett and Molly Nagle contributed to this report.