Washington DC - President Joe Biden hosted congressional leaders at the White House Wednesday to push for fresh aid to Ukraine, with Republicans insisting they must first get major concessions on immigration policy.

Washington's vital military assistance for Ukraine's fight against Russian invasion has dried up.

Republicans are blocking Biden's request for a huge national security package, including $60 billion in aid for Ukraine, assistance for ally Israel, and money that would be spent on reinforcing the US-Mexican border.

Biden has made support for Ukraine following Russia's February 2022 invasion a key plank of his foreign policy and has sought to use it to burnish his global leadership credentials as he seeks reelection in November.

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His likely challenger, Donald Trump, is 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.

"I'm going to tell the president what I'm telling all of you, what we've told the American people: border, border, border," Republican House speaker Mike Johnson told reporters before the meeting, according to US media.

Johnson said his party would not even discuss Ukraine without concessions on immigration, underscoring the deep partisan divides fracturing US politics ahead of the election. The speaker has almost no margin for maneuver because Republicans have only a razor-thin margin in the House, giving the far-right faction control over party policy.

However, there were signs of optimism in the Senate where Democrats hold the thin majority.

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

Biden's press secretary, Karine Jean-Pierre, said the president's focus was on restoring aid to pro-Western Ukraine.

"The president's focus today is going to be about Ukraine and the importance of continuing to support Ukraine, as they fight against tyranny, as they fight against President Putin's aggression," she told reporters.

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Joining Johnson, Schumer, and McConnell at the White House was Democratic House minority leader Hakeem Jeffries. Top members of key Congressional committees were also invited to the meeting, which was to involve a review of classified material.

Biden's chief of staff, Jeff Zients, and National Security Advisor Jake Sullivan were leading the talks on the White House side.

Ukraine has pleaded for more aid as it tries to push back Russia's forces, but growing war fatigue in Western capitals is threatening the pipeline of assistance.