Officials in the ethnically mixed city of Kirkuk in Iraq imposed a nighttime curfew after clashes between Kurds and Turkmen in the oil-rich city. The clashes erupted amid preparations for a controversial referendum on Kurdish independence set to take place next week. A Turkmen official said that gunmen on motorcycles opened fire on an office of the Iraqi Turkmen Front. The guards returned fire, killing one and wounding two of the attackers. He said that hours later a police patrol that included the brother of the man who had been killed attacked another office, which set off clashes. There are Kurds living in Iraq, Iran, Syria, and Turkey and they have long demanded independence. In Iraq, they already have a quasi-state centered around Erbil. Officials in all of the countries where Kurds live oppose the referendum.