The future of US aid for Ukraine hangs in the balance after a last-gasp deal to avoid a government shutdown, despite President Joe Biden's attempts to reassure Kyiv it will get what it needs to fight Russia.

Barely a week after President Volodymyr Zelensky was in Washington appealing for more funds, the compromise struck in Congress late Sunday dropped new funding for Ukraine amid opposition from hardline Republicans.

Biden and his Democratic party say America has a duty to help Ukraine stand up to Russian President Vladimir Putin's brutal invasion, warning that a failure to do so could embolden other autocrats in the future.

But the issue has become so politicized in Washington that the fate of vital military aid is now in jeopardy, just as Kyiv tries to make progress in its sluggish counteroffensive before winter sets in.

Biden urged Republican House Speaker Kevin McCarthy on Sunday to "stop the games" and said he "fully expects" him to secure passage of a separate bill for Ukraine funding soon.

"I want to assure our American allies, the American people, and the people in Ukraine that you can count on our support. We will not walk away," Biden said in an address from the White House.

Ukraine played down the blow, saying Sunday it was "actively working with its American partners" to ensure new wartime aid.

Josep Borrell, the top diplomat for the European Union — a major US partner in delivering aid to Ukraine — said the bloc was "surprised" by the last-minute deal and "regret (the US decision) deeply, thoroughly."

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