Geneva (AFP) - 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 counterprotestors.

The racism committee, part of the UN human rights office, can issue a formal early warning to help prevent "existing problems from escalating into conflict" or to "prevent a resumption of conflict where it has previously occurred", according to the rights office website.

President Donald Trump has been widely criticised for his response to the Charlottesville clashes, after he said "both sides" were to blame for the violence.

The UN committee urged Washington, "as well as high-level politicians and public officials, to unequivocally and unconditionally reject and condemn racist hate speech", without mentioning Trump by name.

"We are alarmed by the racist demonstrations, with overtly racist slogans, chants and salutes by white nationalists, neo-Nazis, and the Ku Klux Klan, promoting white supremacy and inciting racial discrimination and hatred," committee head Anastasia Crickley said in a statement.

The committee monitors compliance with the International Convention on the Elimination of All Forms of Racial Discrimination, which the US ratified in 1994.

The US warning marks the seventh such alert issued in the past decade.

They mainly concern countries gripped by ethnic and religious strife, including Burundi, Nigeria, Iraq and Ivory Coast.