Joe Biden “lost his temper” with Ukrainian President Volodymyr Zelenskyy during a phone call between the pair to discuss military aid, according to reports.
The call, which took place on June 15, reportedly saw Mr Biden raise his voice to Mr Zelenskyy as the Ukrainian President asked for further military assistance.
It came after the US President had just approved an additional £1.5 billion aid to Ukraine as the Russian invasion continued.
Mr Biden told Mr Zelenskyy that the US military were working hard to help Ukraine, urging for his counterpart to show more gratitude, according to NBC News.
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The report said that the relationship between the pair has improved since the phone call.
In recent days, Mr Biden expressed scepticism of Russian President Vladimir Putin's comment in a speech that he has no intention of using a nuclear weapon in Ukraine.
"If he has no intention, why does he keep talking about it? Why is he talking about the ability to use a tactical nuclear weapon?" Mr Biden said in an interview with NewsNation.
"He's been very dangerous in how he's approached this," Mr Biden said.
While the US is preparing a new £237 million package of military assistance for Ukraine to bolster its counter-offensive against Russian forces, a source familiar with the plan said last week.
In an update on the ongoing invasion, Mr Zelenskyy said on Sunday that Ukrainian forces had repelled a "fierce offensive" by Russian troops in the eastern Donetsk region.
Mr Zelenskyy said a military unit from Chop in western Ukraine had undertaken the action, but did not say where the clash had occurred.
"Today they stopped the fierce offensive actions of the enemy," the Ukraine President said in his nightly address.
He added: "The Russian attack was repelled."