Nato leaders gather on Tuesday in Washington for a summit aimed at showing resolve against Russia and support for Ukraine – but the meeting is set to be overshadowed by US President Joe Biden's fight for political survival.

The 81-year-old leader will try to use the three days of pomp marking Nato's 75th anniversary to reassure allies over US leadership, and his own ability to govern, as calls grow for him to quit the fight for a second term in office.

Biden has so far defied pressure from some within his own party to step aside, after a bad debate performance against election rival Donald Trump last month.

"Our allies are looking for US leadership," Biden said in an interview on Monday. "Who else do you think could step in here and do this? I expanded Nato. I solidified Nato."

As doubts swirl over Biden, the rest of the 32-nation alliance is nervously eyeing a potential return to the White House by Trump after elections in November.

On the campaign trail the ex-reality TV star has threatened to blow apart the principle of mutual self-defence that has underpinned Nato since it was founded in the wake of the Second World War.

But it is not just the United States that faces political questions. French President Emmanuel Macron arrives after fending off a push by the country's far-right to win power, new British Prime Minister Keir Starmer has his first international outing, and Hungary's premier Viktor Orban flies in after a much-criticised meeting with Russian leader Vladimir Putin.

Meanwhile, South Korean President Yoon Suk Yeol said he would discuss with Nato leaders the distinct threat North Korea poses to Europe by deepening military ties with Russia, warning that Moscow must choose between the two Koreas where its true interests lie.

It "depends entirely" on Russia where it wants to take future ties with South Korea, Yoon said, adding that Seoul would make a decision on weapons support for Ukraine based on how a new military pact between Moscow and Pyongyang plays out.

"Military co-operation between Russia and North Korea poses a distinct threat and grave challenge to the peace and security on the Korean peninsula and in Europe," Yoon said. (AFP/Reuters)