Tedros Adhanom Ghebreyesus has hit back at US claims the World Health Organization (WHO) has been "co-opted by China".
Dr Tedros, WHO's director-general, was speaking after US Secretary of State Mike Pompeo made the claims in London.
He said such allegations are "untrue and unacceptable" and warned against politicising the coronavirus pandemic.
Pompeo, referring to China at a press conference on Tuesday, said: "You can't go make claims for maritime regions that you have no lawful claim to. You can't threaten countries and bully them in the Himalayas. You can't engage in cover-ups and co-opt international institutions like the World Health Organization."
WHO has been frequently criticised by the US administration over its handling of China during the pandemic.
Washington formally launched a withdrawal procedure from WHO on July 8 after announcing the move in May.
Speaking after Dr Tedros, Maria Van Kerkhove, WHO's COVID-19 technical lead, said: "As an American, I have never been so proud to be a member of this organisation. I witness the work first hand, what Dr Tedros is doing and what our teams are doing around the world."
Trump on Friday cancelled the Republican Party's convention in Florida over concerns about the coronavirus.
It came as the US surpassed a total of 4 million cases.