US President Joe Biden hosts Poland's President Andrzej Duda and Prime Minister Donald Tusk at the White House Tuesday to try to reassure key NATO ally Warsaw of Washington's support after Russia's invasion of Ukraine.

Duda urged NATO allies before the visit to up their spending in response to growing fears about Moscow's aggression, and will reportedly ask Biden to send more US troops to the alliance's eastern flank with Russia.

Alarm is growing among US allies as Congress blocks aid for Ukraine, and after Biden's election rival Donald Trump threatened to cut funding for Kyiv if elected while encouraging Russia to invade NATO countries that fail to pay their dues.

Biden's unusual joint meeting with both hard-right Duda and pro-EU Tusk also underscores US concerns that deep-seated tensions between the Polish leaders could harm the Western alliance.

Biden has made support for Ukraine a key plank of his foreign policy but played down a report that Warsaw would seek an increase in the number of US forces stationed in eastern Europe to guard against Russia.

"There's no need for more troops at the Polish border. But I'm meeting with the Polish leaders tomorrow," Biden told reporters on the eve of the talks.

But the meeting, on the 25th anniversary of the day former Soviet-bloc Poland, the Czech Republic and Hungary joined NATO, will inevitably focus on reassuring Poland of US backing.

"The leaders will reaffirm their unwavering support for Ukraine's defense against Russia's brutal war of conquest," White House Press Secretary Karine Jean-Pierre said in a statement.

Trump's Republicans have however been holding up a crucial multi-billion-dollar military assistance package for Ukraine, allowing Russia to make a series of recent gains.

A US intelligence assessment published Monday said that the "deadlock plays to Russia's strategic military advantages and is increasingly shifting the momentum in Moscow's favor."

'Russian aggression'

Meanwhile in Warsaw, prime minister and former EU chief Tusk won elections in October and has since faced near-daily battles with right-wing president Duda, raising concerns of political instability.

Biden would discuss "democratic values" with the feuding pair, the White House's Jean-Pierre added.

But the two have been relatively unified on Ukraine, which Poland has wholeheartedly supported since Russia's February 2022 invasion, while taking in hundreds of thousands of refugees.

The war has sparked fears in Poland and other eastern European countries that an aggressive Russia could strike a NATO nation next if it is allowed to win in Ukraine.

Biden has also warned that Putin "won't stop" with Ukraine.

Before leaving for Washington, Duda said on Monday that NATO members should increase their defense spending to three percent of GDP in response to Russia's war in Ukraine.

The Western defense alliance currently has a defense spending target of 2 percent of gross domestic product, though Poland already spends around 4 percent and the United States spends 3.5 percent.

Duda said NATO must give a "clear and courageous response to Russian aggression".

"This response will consist of increasing the military capacity of the North Atlantic Alliance," he added.

Kyiv and many Western allies have also been unnerved by the prospect of a White House comeback by Trump in November's US presidential election.

Hungary's Prime Minister Viktor Orban — a Trump ally who is one of the few EU leaders to maintain relations with the Kremlin — said Trump told him during a meeting at the weekend that he would "not give a penny" to the war in Ukraine.