Ukrainian President Volodymyr Zelenskiy's chief of staff said that the postponement of US assistance for Kyiv being debated in Congress would create a "big risk" of Ukraine losing the war with Russia. The remarks by Andriy Yermak were some of the frankest yet from a senior Kyiv official as uncertainty swirls over the future of vital US and European Union assistance packages as Ukraine's war with Russia rages on. If the aid is postponed, "it gives the big risk that we can be in the same position to which we're located now," he said, addressing the audience in English. "And of course, it makes this very high possibility impossible to continually liberate and give the big risk to lose this war." On Monday, White House officials said the US was running out of time and money to help Ukraine fight its war against Russia. President Joe Biden's administration asked Congress in October for nearly $US106 billion ($A162 billion) to fund ambitious plans for Ukraine, Israel and US border security but Republicans who control the House with a slim majority rejected the package. US officials hope they can still get a significant package approved. But on Tuesday, the Republican leader of the US House of Representatives, House Speaker Mike Johnson, responded to the plea by repeating his request for more information about Ukraine strategy and insisting on Republican demands that aid be tied to changes in US immigration policy. The request by House Speaker Mike Johnson comes amid a weeks-long impasse between Democrats and Republicans over security assistance for Kyiv. "I reiterate that President (Joe) Biden must satisfy Congressional oversight inquiries about the Administration's failure thus far to present clearly defined objectives, and its failure to provide essential weapons (for Ukraine) on a timely basis," Johnson said in a letter released on Tuesday. Congress has approved more than $110 billion for Ukraine since Russia's February 2022 invasion, but it has not approved any funds since Republicans took control of the House from Biden's fellow Democrats in January. Biden administration officials were due to conduct classified briefings on the situation in Ukraine for members of Congress on Tuesday. Yermak singled out the threat of no more direct budgetary support as a problem. The Ukrainian government expects to have a $US43 billion ($A66 billion) budget deficit next year. "Of course, without this direct budget support, it will be difficult to keep ... in (the) same positions and... for the people to really survive...during the situation when the war will continue," he said. "That is why it is extremely critically important that this support will be voted and will be voted as soon as possible." Yermak was making his second visit to Washington in a matter of weeks. He said he planned to press lawmakers and administration officials on the critical importance that Congress approve the new aid package. Ukraine conducted a major counteroffensive push this year, but was unable to break through Russian defensive lines. Russia is now on the offensive in the east. Yermak said that Kyiv had a plan for the next year. "We really have a plan and this plan...includes the military operations...includes diplomatic activity and of course it includes our cooperation in the communications and information," he said. Australian Associated Press