A UN committee tasked with combatting racism has issued a formal "early warning" over conditions in the United States, a rare move often used to signal the potential of a looming civil conflict.

The United Nations Committee on the Elimination of Racial Discrimination said it had invoked its "early warning and urgent action procedure" because of the proliferation of racist demonstrations in the US.

It specifically noted the unrest in Charlottesville, Virginia, in which a woman was killed after an avowed white supremacist ploughed his car into a group of anti-racism counter protestors.