Washington DC - Several Republican senators walked out of a classified briefing on Ukraine Tuesday as it descended into a fight over the border crisis after President Volodymyr Zelensky unexpectedly canceled a videolink appearance to appeal for continued US funding.

Zelensky had been due to update the senators on the latest developments in the conflict with Russia and press for them to support a procedural vote expected Wednesday on an emergency aid package that includes more than $60 billion for Kyiv.

The cash has been held up for weeks by a fight in Congress, as the White House has warned that existing funds will run out by the end of the year and that Russia's President Vladimir Putin could win the war if lawmakers fail to act.

Senate Majority Leader Chuck Schumer announced that Zelensky had been prevented from taking part by a "last minute" hiccup but he pressed ahead with the briefing anyway – only for the proceedings to turn into a war of words.

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Utah's Mitt Romney left early, confirming that "a number" of his Republican colleagues had followed suit, angry that they heard nothing on their demand that Ukraine aid be coupled with action on the migrant crisis at the US-Mexico border.

"The briefers were saying things we've all known, we can read about in any newspaper, had been said publicly," Romney told reporters. "There's nothing new in what they're describing, and Republicans are saying that there's support for Ukraine, but there has to be security of our border."

Congress is more divided over backing for Ukraine than it has been at any time during the nearly two-year conflict, with the country fast exhausting the military aid provided by the United States so far.

Senate Republicans are making their support for extra Ukraine funding contingent on President Joe Biden's Democrats accepting reforms of the asylum system and tightened border security – measures the Democrats have already rejected.

"Republicans are just walking out of the briefing because the people there are not willing to actually discuss what it takes to get a deal done," Romney said.

Schumer was quoted by Fox News as saying the briefing had been "immediately hijacked" by Republicans choosing to make a speech on border security rather than asking questions about Ukraine.

One member was "screaming" an admonishment at briefers about not having visited the border, Schumer reportedly said.

The Democrat has teed up a vote Wednesday on clearing the first procedural hurdle for addressing Biden's $106 billion aid request for Ukraine, Israel, and Taiwan.

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But it needs 60 votes in the 100-member Senate, and the 49-strong Republican minority looks likely to defeat the package as it leaves out their immigration reforms.

"The number one most immediate threat to our national security is an open border," Kansas Republican Roger Marshall said outside the briefing room. "Look, everything has been said about Ukraine that can be said. And what's not being said is what's so critical here."

Even if the two sides manage to hammer out a deal in the Senate, it will be a much tougher sell for the Republican-led House, where conservatives have been more skeptical about funding Ukraine, and just as keen to leverage the issue to secure border reforms.

House Speaker Mike Johnson confirmed publicly for the first time in a letter to the White House Tuesday that his party will not pass Ukraine aid unless Congress enacts "transformative change to our nation's border security laws."

But Democrats reacted angrily to what they see as an attempt by Republicans to leverage the conflict to secure domestic priorities.

"I have lots of domestic issues I care about too. I'm not holding Ukraine hostage to the resolution of health care or gun violence," Senator Chris Murphy of Connecticut told reporters.

"They made a choice to put Ukraine funding in jeopardy and they will all have to live with that choice when Vladimir Putin marches into Kyiv and through into Europe."