NATO leaders will end their summit with a clear focus on backing Ukraine and countering what the allies say is the growing threat Russia poses to Europe, as US President Joe Biden faces growing calls from members of his Democratic party to drop his re-election bid.
Biden, 81, will hold a rare solo news conference on Thursday, where reporters are certain to question his candidacy in the November 5 presidential election, despite his hope to shift the narrative, surrounded by allied leaders he has spent his three years in office cultivating.
European leaders worry that November's US presidential election could yield a sharp change in Washington's support for Ukraine and NATO.
Republican candidate Donald Trump, 78, has questioned the amount of aid given to Ukraine to fight Russia's invasion and US support for allies.
Ukrainian President Volodymyr Zelenskiy will also hold a news conference with NATO Secretary-General Jens Stoltenberg on Thursday after meeting Republican and Democratic lawmakers at the US Capitol on Wednesday.
The Ukrainian leader, keen to cement ties with US lawmakers on both sides of the political aisle in the event that Trump is re-elected, met leaders of the Senate and House of Representatives and members of committees involved in defence, spending, diplomacy and national security.
He told reporters he invited Mike Johnson, the Republican leader of the US House of Representatives, to visit Kyiv.
In a clear warning to Russian President Vladimir Putin, the United States on Wednesday said it would start deploying longer-range missiles in Germany in 2026, the most potent US weapons to be based on the European continent since the Cold War.
A NATO declaration also said the allies would provide at least 40 billion euros ($A64 billion) in military aid within the next year, although they stopped short of the multi-year commitment Stoltenberg had sought.
NATO members also pledged to continue to support Ukraine "on its irreversible path to full Euro-Atlantic integration, including NATO membership".
Biden, who will host an event on support for Ukraine, has argued that NATO is "stronger than it's ever been" and Ukraine can and will stop Russian leader Putin "with our full, collective support".
The increasingly embattled US president hosted a dinner for NATO allies and partners at the White House on Wednesday, underscoring the alliance's role in ensuring "freedom, security and democracy" for all its citizens.
He also met new British Prime Minister Keir Starmer, Canadian Prime Minister Justin Trudeau of Canada and Finland's President Alexander Stubb.
While Biden has been seeking to rally allies and Democratic voters, several high-ranking European officials met a top foreign policy adviser to Trump during the summit.
On Wednesday, Trump told Fox News Radio he would not pull the US out of NATO but reiterated that he wanted members to pay more.
"I just want them to pay their bills. We're protecting Europe. They take advantage of us very badly," he said.
Trump had pressed congressional Republicans to stall military aid for Ukraine before later reversing course earlier in 2024.
Biden's uneven June 27 debate performance against Trump and low public approval ratings have raised fresh doubts about his mental fitness, with nine Democratic members of Congress and one Democratic senator calling for him to step aside.
Zelenskiy urged US political leaders in a speech on Tuesday not to wait for the outcome of the US election before moving forcefully to help Ukraine and called for fewer restrictions on the use of US weaponry.
