Islamic State group sleeper cells have assaulted the city of Kirkuk in Iraq, killing at least 19 people, BBC News reported Friday. The militants used rifles, grenades and suicide bombers to storm government buildings in the city, located over 100 miles southeast of the group's embattled stronghold in Mosul.

The group also known as ISIS claimed to have seized the town hall and a central hotel, but city officials denied the reports. At least six police officers and 13 civilians were killed. A state of emergency and curfew were declared.

Multiple suicide bombers attacked three police buildings and a political party headquarters, according to Brig. Gen. Sarhad Qadir, a district police chief. Local police battled ISIS fighters before regaining control. The group also attacked Dibis Power Plant in the north of the city. The site is owned by an Iranian company and is still under construction. Five Iranian employees were among the dead and seven employees and five police guards were wounded.

Gov. Najm al-Din Karim claimed that Kurdish Peshmerga fighters and counter-terrorism forces had successfully defeated all ISIS fighters and Qadir also said all militants within the city had been neutralized, however, there may still have been ISIS fighters fighting in the city’s Dumez district. Gunfire could apparently be heard throughout the city.

At least twelve militants were killed. Karim blamed the attack on sleeper cells within the city and warned of further ISIS action outside of Mosul. "Because of the ongoing Mosul offensive, they may want to create a situation where forces would be withdrawn from there and the focus shifted to Kirkuk," Karim told Kurdish news agency Rudaw. "Also because they are being defeated in Mosul, they want to boost their morale with these kinds of actions."

Iraqi Prime Minister Haider al-Abadi launched an offensive Monday to recapture Mosul with the support of Kurdish forces, a U.S.-led coalition and Shiite militias. Mosul, Iraq’s second-largest city, fell to ISIS in 2014. The anti-ISIS forces have made significant progress in recent days and, while some militants have begun fleeing to Syria, many are trying to hold the line in Iraq.