(AFP) - Ukrainian President Volodymyr Zelensky will press Tuesday for more US military aid to battle Russia, which said any funding would be a "fiasco" and claimed its forces were advancing on the front.

Zelensky, who is in Washington to plead for continued support, will meet with President Joe Biden at the White House and also with congressional leaders from both parties.

For much of the nearly two years that Ukraine has resisted President Vladimir Putin's onslaught, the United States has led a Western coalition sending billions of dollars in weaponry and ammunition.

But Republicans are ever-more openly rejecting the need to fund Ukraine, saying that Biden needs to devote more attention to domestic security, particularly to stopping illegal migration over the US-Mexican border.

The Republicans are also questioning whether Ukraine should keep fighting at all.

Biden is a key supporter who has framed the Ukrainian war effort as part of a global struggle between democracies and aggressive autocracies.

But on Capitol Hill, Zelensky will face his real test when he addresses Republican and Democratic senators, and meets the new Republican speaker of the House, Mike Johnson.

In a speech on Monday at the National Defense University in Washington, Zelensky said that politics should not "betray" Ukraine's soldiers and he echoed Biden in saying that the struggle had global implications.

"When the free world hesitates, that's when dictatorships celebrate and their most dangerous ambitions ripen," he said. "They see their dreams come true when they see delays."

"Putin must lose," he said.

- 'Advanced significantly' -

The Kremlin played down the consequences of any further US funding on Tuesday.

"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.

"The tens of billions of dollars that Ukraine wants to be pumped with, are also headed for the same fiasco."

In an interview with Russian state television on Sunday, Putin said that Ukraine was "running out" of arms because its defense industry was not able to produce enough.

"When there is no base of their own, no ideology of their own, no industry of their own, no money of their own, nothing of their own, then there is no future. And we have one."

Russia further upped the pressure on Tuesday.

"Our units have advanced significantly forward northeast of Novopokrovka," the Moscow-installed head of the Russian-occupied Zaporizhzhia region, Yevgeny Balitsky, said on Telegram.

Ukraine said Russia had launched a "massive offensive" with armoured vehicles in another part of the front near Avdiivka in the east.

And the country's main mobile operator was paralysed by a "powerful hacker attack" which disrupted its services.

The disruptions, which the company's CEO said were an act of "war", came as Ukraine is gearing up for a difficult winter with nightly Russian strikes.

- 'Full mobilisation' -

Kyiv's main ally Poland urged global support for the war-torn country.

Polish prime minister-designate Donald Tusk called for the "full mobilisation on the part of the free world, the West in support of Ukraine in this war."

But Republican senators last week blocked a White House request for $106 billion in emergency aid primarily for Ukraine and Israel.

Conservatives said they would refuse the package for these close foreign allies unless Democrats and the White House also agreed to far-ranging immigration reforms.

One key Republican senator, James Lankford, was quoted by US media on Monday saying "we're not going to be able" to get a deal by the end of this week.

But the Republican right-wing, led by former president and 2024 presidential candidate Donald Trump, has dramatically soured in a much broader sense against Ukraine's cause.

Influential Republicans increasingly question why the United States is supporting what they describe as Ukraine's impossible ambition to drive back all of Russia's invading forces.

"What's in America's best interest is to accept Ukraine is going to have to cede some territory to the Russians and we need to bring the war to a close," Senator JD Vance, a close Trump ally, said Sunday.

He dismissed as "preposterous" White House warnings that allowing Russia to win in Ukraine would put other eastern European countries, including NATO members, at risk.

There should be no "blank check" for Ukraine, Vance said.

"You need to articulate what the ambition is. What is $61 billion going to accomplish that $100 billion hasn't?"

© Agence France-Presse