United Nations, United States, July 18, 2018 (AFP)

US Ambassador Nikki Haley on Wednesday described the Human Rights Council as the "greatest failure" of the United Nations as she defended the US decision to quit the UN body.

Haley told the Heritage Foundation think tank in Washington that the Geneva-based body had "provided cover, not condemnation, for the world's most inhumane regimes."

"Judged by how far it has fallen short of its promise, the Human Rights Council is the United Nations' greatest failure," she said.

The ambassador announced last month that the United States was quitting the council, accusing it of bias against Israel and condemning the "hypocrisy" of its members including China, Egypt, Venezuela and Cuba.

The human rights council has been "not a place of conscience, but a place of politics," Haley said, adding that it had focussed its attention "unfairly and relentlessly on Israel."

US criticism stems from the fact that Israel is the only country that has a dedicated agenda item at council meetings, which means that Israel's treatment of Palestinians has regularly come under scrutiny.

Haley took aim at the council for failing to speak out in support of anti-government protests in Iran, to condemn the violence in Venezuela and over the election of the Democratic Republic of Congo to the council after its security forces faced allegations of atrocities in the Kasai region.

Despite the US exit, "fixing the institutional flaws" of the council remains "one of our biggest priorities" at the United Nations, Haley said.

The withdrawal followed strong UN criticism of President Donald Trump's policy to separate migrant children from their families at the US-Mexico border.

Haley again took a swipe at rights groups like Amnesty International and Human Rights Watch for failing to back the US bid to reform the council through a resolution at the General Assembly.

The non-governmental organizations had instead urged the US administration to seek reforms in Geneva by pushing for more competition in elections for seats at the council.

The United States refused to join the body when it was created in 2006 but joined in 2009 when Barack Obama was in the White House.