US President Joe Biden has assured his Ukrainian counterpart that $60bn in military aid is on its way.
The measure still needs to pass a final congressional vote, but Mr Biden told Volodymyr Zelensky that he was confident it would be approved.
Mr Zelensky had earlier made an urgent appeal for more weapons to avoid a “catastrophic” situation in Europe.
The US has blamed Ukraine’s withdrawal from the battle at Avdiivka on a lack of Congressional support.
Taking Avdiivka – a gateway to the Russian-seized Donetsk regional capital in the east – is Russia’s first notable gain since they seized nearby Bakhmut in May.
“Ukraine’s military was forced to withdraw from Avdiivka after Ukrainian soldiers had to ration ammunition due to dwindling supplies as a result of congressional inaction,” a White House statement read.
Earlier this week, the US Senate approved a $95bn (£75bn) foreign aid package – which includes $60bn for Ukraine – after months of political wrangling.
But it still faces an uphill battle in the House of Representatives, where members of the Republican Party are divided on the measure.
“Look, the Ukrainian people have fought so bravely and heroically, they’ve put so much on the line and the idea that now, when they’re running out of ammunition, we’d walk away – I find it absurd,” Mr Biden told reporters following his call with Mr Zelensky on Saturday.
“I find it unethical, I find it just contrary to everything we are as a country”, he added. “So I’m going to fight until we get them the ammunition they need and the capacity they need to defend themselves.”
The Ukrainian president also urged US lawmakers to approve the financial package.
“I am glad that I can count on the full support of the American president,” he posted on Telegram.
Mr Zelensky has been in Germany, where he made an urgent appeal for more weapons to avoid a “catastrophic” situation in Europe.
“Keeping Ukraine in the artificial deficits of weapons, particularly in a deficit of artillery and long-range capabilities allows Putin to adapt to the current intensity of the war,” he told an international conference in Munich.
“Ukrainians have proven that we can force Russia to retreat,” he said. “We can get our land back.”
“Do not ask Ukraine when the war will end. Ask yourself, why is Putin still able to continue it?”
Ukraine is critically dependent on weapons supplies from the US and other Western allies to keep fighting Russia – a much bigger military force with an abundance of artillery ammunition.
UK Foreign Secretary David Cameron said help for Ukraine from the UK, the EU and the US would make a “real difference” in the fight against Russia.
