The U.N. human rights chief is warning against the "alarming rise in hate speech and incitement to violence against certain ethnic groups" in South Sudan.

Spokeswoman Ravina Shamdasani says Zeid Ra'ad al-Hussein is calling the "hateful rhetoric between Dinkas and Equatorians highly dangerous" and could lead to "mass atrocities if not reined in."

Shamdasani said Tuesday that letters with warnings against ethnic Equatorians have been found outside the offices of several aid groups in Northern Bahr el Ghazal in the country's northwest.

She says ethnic Dinka groups also have warned Equatorians that they will be "eliminated."

Shamdasani says the threats came after an unknown number of Dinka were killed while traveling by bus to the capital, Juba, on Oct. 8.

Fighting continues in South Sudan despite a 2015 peace deal.