"Nobody should be detained or harassed due to the colour of their skin or their nationality, and measures must be taken to protect individuals from acts of violence," said spokesperson Victoria Nuland.
She cited "reports of arbitrary detention and abuse of sub-Saharan African migrants and refugees".
Nuland said the US was continuing to work with other nations to ensure safe passage out of Libya for foreign nationals, including sub-Saharan African migrants, who wish to leave for their own safety.
Amnesty International said on August 31 that people suspected of having fought for former strongman Muammar Gaddafi, in particular black Libyans and sub-Saharan Africans, were at high risk of abuse by anti-Gaddafi forces.
Libya's fledgling new rulers, the National Transitional Council (NTC), have urged their supporters "to treat captives with dignity and to avoid revenge attacks".