President Joe Biden raised his voice at Ukrainian President Volodymyr Zelensky during a June phone call about U.S. aid to Ukraine, NBC reported Monday.

Biden lost his temper during the June 15 call after Zelensky did not appear to show enough gratitude for the aid the U.S. had just sent to Ukraine, the outlet reported, citing four people familiar with the call.

The president had just told Zelensky that he had approved over $1 billion in military aid when Zelensky started speaking about the additional aid Ukraine still needed to defend itself against Russia, according to the outlet. Biden then reportedly raised his voice and told Zelensky he could show more gratitude for everything the U.S. was already doing. (RELATED: Biden To Give Ukraine $1.2 Billion In New Aid As Invasion Continues)

Biden’s frustration with Zelensky had been growing for weeks before the June call, three anonymous sources told NBC News, as the president felt like he was working as hard as he could to deliver aid while the Ukrainian president was focusing too much on what hadn’t yet been done.

The Zelensky team decided to backpedal and ease tensions with Biden after the call, the outlet reported, and the Ukrainian president released a public statement thanking the United States.

“I had an important conversation with U.S. President Biden today,” Zelensky said at the time. “I am grateful for this support. It is especially important for our defense in Donbas.”

In the lead up to the midterms, Biden has expressed concern that Republicans will cut back funding for Ukraine if they win back the House and Senate.

“They said that if they win, they’re not likely to fund to help — continue to fund Ukraine and the Ukrainian war against the Russians. These guys don’t get it. It’s a lot bigger than Ukraine. It’s Eastern Europe. It’s NATO. It’s real serious, serious, consequential outcomes. They have no sense of American foreign policy,” Biden said while speaking at a reception for Democratic Pennsylvania Senate candidate John Fetterman.