The United States has withdrawn from the United Nations Human Rights Council, accusing it of a "chronic bias against Israel".
Activists have warned that the move would make advancing human rights globally even more difficult.
Standing with US Secretary of State Mike Pompeo, the US Ambassador to the UN Nikki Haley slammed Russia, China, Cuba and Egypt for thwarting US efforts to reform the council.
She also criticised countries that shared US values and encouraged Washington to remain but "were unwilling to seriously challenge the status quo".
The United States is half-way through a three-year term on the main UN rights body and the Trump administration had long threatened to quit if the 47-member Geneva-based body was not overhauled.
"Look at the council membership, and you see an appalling disrespect for the most basic rights," said Ms Haley, citing Venezuela, China, Cuba and Democratic Republic of Congo.
She also said the "disproportionate focus and unending hostility toward Israel is clear proof that the council is motivated by political bias, not by human rights".
UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein has voiced dismay at the decision by the US, and said that Washington should step up its engagement given the number of violations worldwide.
The US withdrawal is its latest rejection of multilateral engagement after it pulled out of the Paris climate agreement and the 2015 Iran nuclear deal.
It also comes as the United States faces intense criticism for detaining children separated from their immigrant parents at the US-Mexico border.
Earlier this week, Mr Zeid called on Washington to halt its "unconscionable" policy.
Rights groups have criticised the Trump administration for not making human rights a priority in its foreign policy.
Critics say this sends a message that the administration turns a blind eye to human rights abuses in some parts of the world.
Diplomats have said the US withdrawal from the body could bolster countries such as Cuba, Russia, Egypt and Pakistan, which resist what they see as UN interference in sovereign issues.
Among reforms the US had been seeking was to make it easier to kick out member state with egregious rights records.
Ms Haley said the US withdrawal from the Human Rights Council "is not a retreat from our human rights commitments".