The former United Nations human rights envoy to Afghanistan, Cherif Bassiouni, has said he lost his job because of pressure from the United States.
Bassiouni criticised US military policing
The UN Human Rights Commission ended Professor Bassiouni's mandate at a meeting in Geneva last week.
American officials said Afghanistan's human rights situation had improved.
But Prof Bassiouni said it was because US defence officials did not want investigations into the way people were detained without trial by US forces.
Prof Bassiouni has spent the past year investigating allegations of human rights violations in Afghanistan for the commission.
The professor, who was nominated for the Nobel Peace Prize in 1999, was extremely critical of Washington's policy on detainees.
Last week the commission - made up of 53 elected member states - decided not to renew his mandate.
Professor Bassiouni, an Egyptian-born law professor at DePaul University in Chicago, had pressed for access to US detention facilities.
He had also criticised the conditions in which many detainees were held, both by American-led coalition troops and Afghan forces.
Unnamed American officials have been quoted in the US media as saying the decision over Prof Bassiouni was partially because of improvements in Afghanistan's human rights situation.
But in an interview with the BBC, Prof Bassiouni alleged there was an intensive lobbying campaign by US officials in Geneva.
"It has nothing to do with the work in Afghanistan or the situation in Afghanistan," he said.
"This is a very narrow, limited issue that is of concern to the US Defence Department and the hawks in the administration who simply do not want anybody to look into the way people are being detained in Afghanistan by US forces."
Prof Bassiouni said the commission possibly bowed to US pressure in return for US support or concessions on other resolutions.
Jose Diaz, spokesman for the Office of the UN High Commissioner for Human Rights, said the office would continue to monitor the human rights situation in Afghanistan and report publicly about it.
There would still be public, international scrutiny of the human rights situation in Afghanistan, he said