Washington DC - The White House urged US senators Thursday to reach a deal linking vital Ukraine aid to border security after reports that former president Donald Trump, who is seeking reelection, was trying to torpedo an agreement.

US media reported that Trump had been urging Republicans to oppose the deal, partly to deny Democratic President Joe Biden a political win ahead of November's vote.

A deal between Republicans and Democrats in the Senate tying military assistance for Kyiv with money to counter immigration at the Mexican border had seemed on the cards just days ago.

Money for US aid to Ukraine has run out due to the lack of an agreement in Congress, leaving the key US ally short of ammunition as it tries to fight off Russia's invasion.

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White House Deputy Press Secretary Olivia Dalton said Biden's administration had been working in "good faith" with Republicans to reach a deal, including a meeting with the president last week.

"We're going to continue to work on that in good faith, and we hope that Republicans will remain at the table so we can do that," she told reporters aboard Air Force One.

National Security Council spokesman John Kirby said Russia was "watching closely" to see whether Washington would continue to deliver support for Ukraine.

"Ukraine is heading into a critical few months here as winter is full on and spring approaches, and the Russians have shown no intent to slacken off from the drone and missile attacks," he said.

Republican Senate Minority Leader Mitch McConnell reportedly told senators in a closed-door meeting on Wednesday that they should not "undermine" Trump, US media outlets reported. Discussions were continuing Thursday, they said.

Trump has made immigration a major plank of his campaign for a return to the White House and has long opposed more aid for Ukraine.

Trump was impeached while in office after linking Ukraine's self-defense aid to demands for politically damaging material on Biden and his son's business deals there.