BAGHDAD -- Iraq's ethnically-mixed and contested city of Kirkuk was on a nighttime curfew on Tuesday after clashes erupted there the previous night between Kurds and Turkmen amid preparations for the controversial Kurdish independence referendum next week, a local Turkmen official said.

The Iraqi Kurds plan to hold the referendum on Sept. 25 in three governorates that make up their self-ruled region, as well as in disputed areas that are controlled by Kurdish forces but claimed by Baghdad, including the oil-rich province of Kirkuk.

Baghdad, Turkey, Iran and the international community have rejected the vote and asked the Kurds to call it off to avoid further destabilizing the region.

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Shortly after sunset Monday, gunmen on motorcycles opened fire on one of the offices of the Iraqi Turkmen Front, Mohammed Samaan Kanaan, in charge of the Front's offices, told The Associated Press over the phone. The guards returned fire, killing one and wounding two of the assailants, Kanaan added.

Hours later, a police patrol that included the brother of the slain assailant attacked another office, triggering clashes, Kanaan said. The fighting ended when a large ethnically mixed force reached the scene. No casualties occurred in the second bout of clashes.

Local Kurdish officials and officials in Baghdad were not immediately available to comment on the clashes or the curfew.

Kirkuk is home to Arabs, Kurds, Turkmen and Christians. Kurdish forces took control of the province and other disputed areas in the summer of 2014, when the Islamic State of Iraq and Syria (ISIS) militants swept across northern and central Iraq and the Iraqi armed forces crumbled.

On Monday, Iraq's top court temporarily suspended the northern Kurdish region's referendum on independence, saying it "issued a national order to suspend the referendum procedures ... until the resolution of the cases regarding the constitutionality of said decision."

The move is just the latest in a number of rulings from Iraq's central government attempting to stop the vote. On Sept. 12, Iraq's parliament voted to reject the referendum and on Sept. 14, lawmakers voted to dismiss the ethnically mixed Kirkuk province's Kurdish governor who supports the referendum.

Despite strong opposition from Baghdad, regional leaders and the United States - a key ally of Iraq's Kurds - Kurdish officials have continued to pledge that the vote will be held.