WASHINGTON, United States - President Joe Biden warned Republicans Wednesday that blocking vital US military aid for Ukraine threatened the "free world" as talks with congressional leaders at the White House failed to produce a breakthrough.

US President Joe Biden steps off Air Force One after returning from Philadelphia where he participated in a service event as the country marks Martin Luther King, Jr. Day on January 15, 2024 at Joint Base Andrews in Maryland. (Photo by Mandel NGAN / AFP)

US assistance for Ukraine has dried up with Republicans refusing to approve Biden's request for $60 billion for Kyiv until the Democrat bows to their demands for measures to curb migration over the Mexican border.

The bitter partisan battle has been intensified by the looming US presidential election in November, and has raised deep concerns among Western allies who fear it could harm Kyiv's fight against Russia's invasion.

Biden told the visiting House and Senate leaders that the months-long standoff "endangers the United States' national security, the NATO alliance, and the rest of the free world," the White House said.

"The president called on Congress to quickly provide additional funding to support Ukraine and send a strong signal of US resolve. The president also made clear that we must act now to address the challenges at the border."

Republican House speaker Mike Johnson said it was a "productive meeting" but insisted Biden needed to do more on migration.

"We must insist that the border be the top priority. I think we have some consensus around that table," he told reporters outside the West Wing after the meeting.

Democratic Senate leader Chuck Schumer said there was "broad agreement in the room" that the two parties had to find a bipartisan solution.

"The president himself said over and over again that he is willing to move forward on the border," Schumer told reporters.

Schumer and his Republican counterpart Mitch McConnell had both indicated that a vote on Ukraine aid, along with changes to immigration policy, was a growing possibility, US media reported.

- 'Fight against tyranny' -

Biden's chief of staff Jeff Zients, National Security Advisor Jake Sullivan and Director of National Intelligence Avril Haines also took part in the talks on the White House side.

Biden has warned that Russian President Vladimir Putin will benefit from the chaos over Ukraine funding, saying it could encourage Moscow and other authoritarian regimes to make territorial grabs.

The Democrat has made support for Ukraine a key plank of his foreign policy and has sought to use it to burnish his global leadership credentials as he seeks reelection later this year.

He bundled the Ukraine aid together with money for Israel's war against Hamas and measures to strengthen the Mexican border as part of a huge $110 billion package he proposed to Congress in October.

His likely challenger, Donald Trump, is meanwhile leading hardline Republican moves to put illegal immigration over the Mexico border at the center of the election -- and to use Ukraine as leverage to force change.

The aid blockage has been compounded by Republican in-fighting, with speaker Johnson having almost no margin for maneuver due to a razor-thin margin in the House, giving the far-right faction control over party policy.

US lawmakers are simultaneously negotiating a deal to agree on temporary funding and avoid a government shutdown on Friday.

Ukraine has pleaded for more aid as it tries to push back Russia's forces, but after a failed counteroffensive last year it faces growing fatigue not just in Washington but in other Western capitals.