Washington (AFP) - NATO leaders gather 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 disastrous TV debate against election rival Donald Trump last month threw into stark relief fears that he lacks mental acuity and physical fitness.

"Our allies are looking for US leadership," Biden said in an interview Monday. 

"Who else do you think could step in here and do this? I expanded NATO. I solidified NATO."

Other leaders look keen to rally around their host, with German Chancellor Olaf Scholz insisting as he set off from Berlin on Tuesday that he was "not concerned" by the president's fitness. 

But as doubts swirl over Biden, 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 volatile ex-reality TV star has threatened to blow apart the principle of mutual self-defense that has underpinned NATO since it was founded in the wake of World War II.

It's not just the United States, however, that faces political questions. 

French President Emmanuel Macron is struggling to form a government after divisive elections, new British Prime Minister Keir Starmer makes his first international outing, and Hungary's premier Viktor Orban flies in after a much-criticized meeting with Russian leader Vladimir Putin.

Air defences for Ukraine

While they wrestle with the minefield of US politics, NATO leaders will have to show they haven't been distracted from the reality of the battlefield in Ukraine. 

Ukrainian President Volodymyr Zelensky will fly in Tuesday expecting to secure additional Patriot advanced air defense systems that he has been begging his backers to send for months to stave off Russian attacks.

His war-torn country's vulnerability to Moscow's missiles was cruelly exposed by a strike Monday on a children's hospital in Kyiv.  

US National Security Council spokesman John Kirby told CNN that NATO leaders would "unify over continued air defense capabilities for Ukraine."

"We've got to make sure that they have a better means and more of it to defend themselves," he said. 

The Kremlin said it was following the summit "with the greatest attention... the rhetoric at the talks and the decisions that will be taken and put on paper."

The promise of more weaponry is set to be the biggest win the Ukrainian leader will get as his troops struggle to hold ground two and a half years into Russia's invasion.

- China challenge -

Worried about dragging NATO closer to war with Russia, the United States and Germany have shut down any talk of giving Ukraine a clear invitation to join their alliance. 

Instead diplomats say Kyiv's path to eventual membership may be described as "irreversible" in the summit declaration and say the country is on a "bridge" to joining. 

NATO members will vow to keep supporting Ukraine at the rate they have been so far since Moscow invaded -- roughly 40 billion euros annually -- for at least another year.

They will also agree the alliance will take more control of coordinating weapon deliveries to Ukraine from the US military in a move to help insulate supplies from any changes in Washington if Trump wins the election. 

While NATO views Russia as it's main threat, it is also paying greater attention to the challenge from China and accuses Beijing of playing a key role in keeping Moscow's war effort going by supplying tech to the Kremlin's military. 

The leaders of Australia, Japan, New Zealand and South Korea will come to Washington to bolster ties with the alliance.

China's foreign ministry hit out at the "smear and attacks" against Beijing from NATO and said the alliance was seeking an excuse to expand its influence in the Asia-Pacific region.