US envoy Nikki Haley has attacked rights groups for trying to "thwart reform" of the United Nations Human Rights Council (UNHRC).
In a letter, Ms Haley said groups worked "opposite the United States" and were partially to blame for the US withdrawal from the body.
The country withdrew from the UNHRC on Tuesday after accusing it of bias.
Her letter was sent to the International Humanist and Ethical Union, which represents 162 groups.
Ms Haley berated the groups for not supporting proposed US changes to the UNHRC, saying they "sought to undermine" them and worked "on the side of Russia and China, and opposite the United States".
The letter says they were a contributing factor to the US withdrawal from the council due to their attempts "to block negotiations and thwart reform".
The US pulled out of the UNHRC on Tuesday, calling it a "cesspool of political bias".
Announcing the decision to leave the council, Ms Haley described it as a "hypocritical and self-serving organisation" that displayed "unending hostility towards Israel".
She was speaking alongside US Secretary of State Mike Pompeo, who denounced the council as "a protector of human rights abusers".
The US pulled out of the UN's cultural organisation Unesco in 2017, again for perceived bias against Israel.