President Joe Biden said Sunday that American aid to Ukraine will keep flowing for now as he sought to reassure allies of continued U.S. financial support for the war effort. But time is running out, the president said in a warning to Congress.

"We cannot under any circumstances allow America's support for Ukraine to be interrupted," Biden said in remarks from the Roosevelt Room after Congress averted a government shutdown by passing a short-term funding package late Saturday that dropped assistance for Ukraine in the battle against Russia.

"We have time, not much time, and there's an overwhelming sense of urgency," he said, noting that the funding bill lasts only until mid-November. Biden urged Congress to negotiate an aid package as soon as possible.

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"The vast majority of both parties — Democrats and Republicans, Senate and House — support helping Ukraine and the brutal aggression that is being thrust upon them by Russia," Biden said. "Stop playing games, get this done.''

But many lawmakers acknowledge that winning approval for Ukraine assistance in Congress is growing more difficult as the war grinds on. Republican resistance to the aid has been gaining momentum and the next steps are ahead, given the resistance from the hard-right flank.

While Senate Majority Leader Chuck Schumer, D-N.Y., has begun a process to potentially consider legislation providing additional Ukraine aid, House Speaker Kevin McCarthy, R-Calif., faces a more difficult task in keeping the commitment he made over the objections of nearly half of his GOP majority.

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He told CBS' "Face on the Nation" that he supported "being able to make sure Ukraine has the weapons that they need," but that his priority was security at the U.S.-Mexico border.

"I firmly support the border first," he said. "So we've got to find a way that we can do this together."

By omitting additional Ukraine aid from the measure to keep the government running, McCarthy closed the door on a Senate package that would have funneled $6 billion to Ukraine, roughly one-third of what has been requested by the White House. Both the House and Senate overwhelmingly approved the stopgap measure, with members of both parties abandoning the increased aid in favor of avoiding a costly government shutdown.

Now Biden is working to reassure U.S. allies that more money will be there for Ukraine.

"Look at me," he said turning his face to the cameras at the White House. "We're going to get it done. I can't believe those who voted for supporting Ukraine -- overwhelming majority in the House and Senate, Democrat and Republican -- will for pure political reasons let more people die needlessly in Ukraine."

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Foreign allies, though, were concerned. European Union foreign policy chief Josep Borrell said Sunday from Kyiv that he believed it wouldn't be the last word, but he noted the EU's continued substantial financial support for Ukraine and a new proposal on the table.

"I have a hope that this will not be definitive decision and Ukraine will continue having the support of the U.S.," he said.

The latest actions in Congress signal a gradual shift in the unwavering support that the United States has so far pledged Ukraine in its fight against Russia, and it is one of the clearest examples yet of the Republican Party's movement toward a more isolationist stance. The exclusion of the money for Ukraine came little more than a week after lawmakers met in the Capitol with Ukrainian President Volodymyr Zelenskyy. He sought to assure them that his military was winning the war, but stressed that additional assistance would be crucial.

After that visit, Schumer said that one sentence summed up Zelenskyy's message in his meeting with the Senate: "'If we don't get the aid, we will lose the war," Schumer said.

McCarthy, pressured by his right flank, has gone from saying "no blank checks" for Ukraine, with the focus being on accountability, to describing the Senate's approach as putting "Ukraine in front of America."

The next funding deadline, which comes during the U.S.-hosted meeting in San Francisco of Asia-Pacific Economic Cooperation leaders, is likely to become a debate over border funding in exchange for additional Ukraine aid.

This was the scenario that Mitch McConnell, the Senate Republican leader who has championed Ukraine aid, was trying to avoid back in summer when he urged the White House team not to tangle the issue in the government shutdown debate, according to people familiar with his previously undisclosed conversations with the administration who spoke on condition of anonymity to discuss the private talks. Now, all sides are blaming the other for the failure, straining to devise a path forward.

Voting in the House this past week pointed to the potential trouble ahead. Nearly half of House Republicans voted to strip $300 million from a defense spending bill to train Ukrainian soldiers and purchase weapons. The money later was approved separately, but opponents of Ukraine support celebrated their growing numbers.

The U.S. has approved four rounds of aid to Ukraine in response to Russia's invasion, totaling about $113 billion, with some of that money going toward replenishment of U.S. military equipment that was sent to the front lines. In August, Biden called on Congress to provide for an additional $24 billion.