At least 12 people, including eight foreign employees of the United Nations, have been killed in northern Afghanistan, after a protest against the burning of the Quran turned violent, Afghan police said Friday.

U.N. Secretary-General Ban Ki-moon condemned as “cowardly” the attack on the U.N. mission ‘s compound in Mazar-e-Sharif, the capital of Balkh province.

Afghan officials said demonstrators stormed the U.N. office during protests against the recent burning of the Quran by an American preacher in the southern U.S. state of Florida.