President Joe Biden thanked Prime Minister Mette Frederiksen on Monday for Denmark's role in a Western alliance "standing up" for Ukraine as it tries to fend off Russia's 15-month-old invasion.

The Oval Office visit was the first of a pair of critical meetings Biden is holding with European allies this week that will focus heavily on what lies ahead in the war in Ukraine — including the recently launched effort to train, and eventually equip, Ukraine with American-made F-16s fighter jets. Biden will meet Thursday with British Prime Minister Rishi Sunak.

Both Britain and Denmark are playing a pivotal role in the nascent joint international plan that Biden recently endorsed after months of resisting calls from Ukrainian President Volodymyr Zelenskyy for U.S. aircraft.

"There's a shared commitment to the core values and that gives us our strength — at least that's what I believe," Biden said. "Together we're working to protect those values — including standing up for the people of Ukraine against the brutal aggression of the Russians."

Biden's separate meetings with the leaders of two key NATO allies come at a crucial period in the 15-month war as Ukraine readies to launch a counteroffensive. It's also a moment when the U.S. and Europe are looking to demonstrate to Moscow that the Western-alliance remains strong and focused on cementing a longer-term commitment to Ukraine with no end to the conflict in sight.

Neither mentioned the F-16 agreement in their brief remarks before reporters at the start of the meeting, but White House officials said that coordination on training Ukraine on modern aircraft would be a central part of the talks.

Frederiksen for her part thanked Biden for leading the transatlantic alliance.

"I am looking forward to working even closer with you on defense and security," she said.

Denmark has purchased dozens of American-made F-16s since the 1970s and has indicated it is open to the possibility of providing Ukraine with some. Britain strongly advocated for a coalition to supply Ukraine with fighter planes, and says it will support Ukraine getting the F-16s it wants. But the U.K. does not have any F-16s, and has ruled out sending Royal Air Force Typhoon jets.

Instead, Britain says it will give Ukrainian pilots basic training on Western-standard jets starting in early summer to prepare them to fly F-16s. The Ukrainian pilots will then go on to other countries for the next stages of training.

The F-16 agreement is among several recent high-profile efforts by the U.S. and Europe focused on bolstering Western resolve as the war grinds on. Moscow officials claimed that Ukrainian forces were making a major effort to punch through Russian defensive lines in southeast Ukraine for a second day Monday. Kyiv authorities didn't confirm the attacks and suggested the claim was a Russian misinformation ruse.

Asked by a reporter during his Oval Office meeting with Frederiksen whether he believed the Ukrainians would be successful in their counteroffensive, Biden said nothing but raised his hand and crossed his fingers.

Last week, Frederiksen and Sunak were among 45 European leaders who traveled to Moldova for the first summit of the European Political Community where they underscored support for Eastern Europe's ambitions to draw closer to the West and keep Moscow at bay.

Biden was also expected to discuss with Frederiksen and Rishi preparations for next month's NATO summit in Lithuania that comes amid growing pressure on the alliance from Zelenskyy on NATO to offer Ukraine concrete security guarantees and a defined path for Kyiv to eventually win membership into the group.

The 31-member alliance is also looking at boosting Ukraine's non-member status in NATO and preparing a framework for security commitments that it can offer once the war with Russia is over.

Max Bergmann, a former senior State Department official during the Obama administration, said Biden and his European counterparts' task is to stay on the same page for what comes after Ukraine's much-anticipated counteroffensive.

"Throughout this conflict, we have not only underestimated the Ukrainians but we have also underestimated the Europeans," said Bergmann, who is now director of the Europe, Russia and Eurasia Program at the Center for Strategic and International Studies in Washington. "They're not wavering but they will also need to keep finding new funds to plow into military equipment to support the Ukrainians. There's a question on both sides of the Atlantic: How much will it actually take to sustain Ukraine?"

Following the meeting, Frederiksen suggested in an exchange with reporters that she had dim hopes of an endgame to the conflict in the near term.

"It takes two to tango," she said. "So we need some signals from Russia and I don't think any of us in the alliance are willing to do anything without Ukraine. So it starts with them — what Ukraine wants out of this."

Denmark's centrist government said last week that it would look to invest some 143 billion kroner ($20.6 billion) in the country's defense over the next decade, citing a "serious threat picture." The government has an ambition to reach NATO's target of spending 2% of gross domestic product on military budgets by 2030, in part as a response to Russia's invasion of Ukraine.

The spending plan was announced as Frederiksen has emerged as a possible contender to succeed NATO Secretary-General Jens Stoltenberg when his term ends in September. Asked whether she spoke to Biden about the expected vacancy, Frederiksen said she did not want to go "further in these speculations about NATO."