Ukrainian President Volodymyr Zelensky held last-ditch talks in Washington to plead for continued US aid on Tuesday, as Russia claimed advances on the battlefront and dismissed the impact of Western help for Kyiv.

After Zelensky met the leaders of both parties in the US Senate, Democratic Majority leader Chuck Schumer warned that Ukraine needed the aid quickly to prevent Russian President Vladimir Putin claiming victory.

Zelensky then went on to the House of Representatives to confer with the Republican speaker, Mike Johnson, and planned to move to the White House for one-on-one talks with President Joe Biden.

"He made it clear and we all made it clear that if we lose, Putin wins," Schumer said after the talks with Zelensky, who wore a black sweater with a small Ukrainian trident symbol, and olive green military trousers.

The trip -- coming as Ukraine's main phone operator suffered what it said was a massive hacking attack -- represents a desperate bid by Zelensky to keep Ukraine's key international backer from drifting away.

But while Zelensky won applause from senators, Republicans are increasingly lukewarm about the need to support Ukraine.

They are blocking Biden's request for US$60 billion in new military aid unless he agrees to measures on US-Mexico border security and immigration reform -- one of the most intractable issues in US politics.

The White House has warned that money for Ukraine will run out by the end of the year without a deal with Congress.

In a speech on Monday to US military officials, Zelensky said that politics should not "betray" Ukraine's soldiers and he echoed Biden's frequent warning that the Ukrainian struggle against invasion has global implications.

The Kremlin scoffed at the impact of US support, echoing the arguments made by some senior Republicans who say that continuing the flood of weaponry to Ukraine would be futile after Kyiv's summer counteroffensive stalled.

"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 spokesman Dmitry Peskov said Tuesday.

Putin said on Sunday that Ukraine was "running out" of arms because its defence industry was not able to produce enough.

Russia said it was pressing ahead on the ground, just as Ukraine's freezing winter deepens. (AFP)