Rupert Colville, U.N. human rights spokesman, also called on authorities in Iran to restore the internet service cut off since Saturday and uphold the demonstrators' rights to freedom of expression and peaceful assembly.
It had reports that the number killed was in the dozens, he told a Geneva briefing, adding that the extent of casualties was "clearly very serious."
Iran's Supreme Leader Ayatollah Ali Khamenei on Sunday blamed the turmoil on Iran's foreign foes, including the United States, and denounced protesters as "thugs."