South Korean president Yoon Suk-yeol was caught on a hot mic insulting American lawmakers as "idiots" for failing to pass the additional $6 billion (£5.3 billion) in aid to the United Nations Global Fund.
He was overheard after his brief meeting with US President Joe Biden at the Global Fund in New York.
In the footage published on the official YouTube channel of South Korean television MBC, Yoon can be clearly seen walking along the stage after chatting with Biden before turning to his aides and speaking.
“What an embarrassment for Biden, if these idiots refuse to grant it in Congress,” a video broadcast on South Korean television showed Yoon telling Foreign Minster Park Jin about Biden's drive to increase the US contribution to Global Fund, which would require congressional approval.
The footage immediately went viral in South Korea, garnering five million views in just one day.
At the conference, Biden had pledged $6 billion from the US to be funded after the approval by Congress, for the public health campaign to fight AIDS, tuberculosis, and malaria worldwide.
However, the presidential office of South Korea denied that the remarks were targeted at the US, and his spokeswoman Kim Eun-hye said he was misheard for a similar-sounding Korean word and he was referring to South Korea’s parliament instead of the US.
Park Hong-geun, the South Korean opposition leader, did not accept the justification as he criticized Yoon for hampering the US-South Korean ties.
He called it “an ill-founded excuse” by the presidential office trying to cover up a “diplomatic disaster.”
"I've listened to it 10 times now. It's 'Biden' for sure," one commentator wrote on YouTube.
The opposition said Yoon's official denial was akin to telling South Koreans they were "hearing impaired", Democratic Party MP Chun Jae-soo said in a radio interview on Friday.
With around 27000 of its troops stationed in South Korea, the US is South Korea’s key security ally.
The conference came as the US signed the Inflation Reduction Act (IRA) that will eliminate consumer tax credits for South Korean automakers without operational electric vehicle plants in the United States.
The IRA is stated to have caused an unexpected discord in relations between the two allies because of the new rules on subsidies for EVs from the Biden administration’s side.
South Korea is seeing the new act as a violation of trade rules.
“President Yoon asked for close cooperation so that the U.S. administration can resolve our concerns in the process of enforcing the Inflation Reduction Act,” Yoon’s office said in a statement.