The US ratified the International Convention on the Elimination of All Forms of Racial Discrimination in 1994. By ratifying the convention the US agreed to the convention as outlined below. If the US is, or is about to break the convention the UN should give advice but it would appear that there are limits in what the UN can do.
From the UN
""23 August 2017 – The United Nations body monitoring implementation of the global convention on prohibiting racial discrimination has called on high-level politicians and public officials of the United States to unequivocally and unconditionally reject and condemn racist hate speech and crimes in Charlottesville and throughout the country.
“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,” said Anastasia Crickley, Chairperson of UN Committee on the Elimination of Racial Discrimination (CERD), in a news release.
In a decision issued under its 'early warning and urgent action' procedure, the Committee, which monitors implementation of the International Convention on the Elimination of All Forms of Racial Discrimination, stated “there should be no place in the world for racist white supremacist ideas or any similar ideologies that reject the core human rights principles of human dignity and equality.”
The United States ratified the Convention in 1994""
http://www.un.org/apps/news/story.asp?NewsID=57399#.Wa-6KdGQzcs
""United States of America
Upon signature:
"The Constitution of the United States contains provisions for the protection of individual rights, such as the right of free speech, and nothing in the Convention shall be deemed to require or to authorize legislation or other action by the United States of America incompatible with the provisions of the Constitution of the United States of America."
Upon ratification:
"I. The Senate's advice and consent is subject to the following reservations:
(1) That the Constitution and laws of the United States contain extensive protections of individual freedom of speech, expression and association. Accordingly, the United States does not accept any obligation under this Convention, in particular under articles 4 and 7, to restrict those rights, through the adoption of legislation or any other measures, to the extent that they are protected by the Constitution and laws of the United States.
(2) That the Constitution and laws of the United States establish extensive protections against discrimination, reaching significant areas of non-governmental activity. Individual privacy and freedom from governmental interference in private conduct, however, are also recognized as among the fundamental values which shape our free and democratic society. The United States understands that the identification of the rights protected under the Convention by reference in article 1 to fields of `public life' reflects a similar distinction between spheres of public conduct that are customarily the subject of governmental regulation, and spheres of private conduct that are not. To the extent, however, that the Convention calls for a broader regulation of private conduct, the United States does not accept any obligation under this Convention to enact legislation or take other measures under paragraph (1) of article 2, subparagraphs (1) (c) and (d) of article 2, article 3 and article 5 with respect to private conduct except as mandated by the Constitution and laws of the United States.
(3) That with reference to article 22 of the Convention, before any dispute to which the United States is a party may be submitted to the jurisdiction of the International Court of Justice under this article, the specific consent of the United States is required in each case.
II. The Senate's advice and consent is subject to the following understanding, which shall apply to the obligations of the United States under this Convention:
That the United States understands that this Convention shall be implemented by the Federal Government to the extent that it exercises jurisdiction over the matters covered therein, and otherwise by the state and local governments. To the extent that state and local governments exercise jurisdiction over such matters, the Federal Government shall, as necessary, take appropriate measures to ensure the fulfilment of this Convention.
III. The Senate's advice and consent is subject to the following declaration:
That the United States declares that the provisions of the Convention are not self-executing."
""
https://treaties.un.org/Pages/ViewDetails.aspx?src=IND&mtdsg_no=IV-2&chapter=4&clang=_en