Washington DC - President Joe Biden warned that Russia was banking on the US abandoning Ukraine as he promised President Volodymyr Zelensky he would support Kyiv despite Republicans blocking new military aid.

Standing alongside Biden at a press conference at the White House, Zelensky pledged for his part that Ukraine would keep fighting and said it would be "insane" for Kyiv to give up any territory to secure a peace deal.

Russian President Vladimir Putin "is banking on the United States failing to deliver for Ukraine. We must, we must, we must prove him wrong," said Biden. He said letting Ukraine lose would embolden Putin "and would-be aggressors everywhere."

Biden stressed that he would "not walk away from Ukraine," adding that the US would "continue to supply Ukraine with critical weapons and equipment as long as we can."

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But just how long the Biden administration can keep doing so is under question. Republicans in Congress refuse to pass $60 billion in fresh Ukraine aid if Democrats don't agree to major immigration reforms.

Zelensky, who spent the morning talking to Republicans and Democrats in Congress, signaled cautious optimism that the stalled US aid flow will restart.

"I got the signals. They were more than positive. But we know that we have to separate words and particular results," Zelensky said.

The Ukrainian leader also dismissed suggestions he could concede territory taken by Russia since its February 2022 invasion to move any ceasefire closer.

"That's insane, to be honest," he said, before setting out plans to use Western aid to achieve air superiority over Ukraine and to attack Russian naval assets in 2024.

Biden said he had approved a further $200 million in US military aid to tide Ukraine over for coming weeks, with the White House warning that funding could dry up by the end of the year without a deal in Congress.

House Speaker Mike Johnson expressed little enthusiasm for approving Biden's request, despite his meeting with Zelensky.

"What the Biden administration seems to be asking for is billions of additional dollars with no appropriate oversight, no clear strategy to win, and none of the answers that I think the American people are owed," Johnson said.

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The Kremlin echoed Republican arguments, scoffing at the impact of US support.

"It is important for everyone to understand: the tens of billions of dollars pumped into Ukraine did not help it gain success on the battlefield," Kremlin spokesperson Dmitry Peskov said.

And Russia said it was pressing ahead on the ground, just as Ukraine's freezing winter deepens and Moscow's air attacks on Ukraine's cities increase.