Gunmen stormed into a Baghdad mall on Monday after setting off a car bomb and launching a suicide attack at its entrance, killing at least 18 people and wounding 50 in the city's mainly Shiite east, Iraqi officials said.
ISIS claimed responsibility for the attack in an online statement.
The Iraqi officials initially described the attack as a hostage situation, estimating that 50 people were trapped inside the complex. But Iraqi forces soon surrounded the building and landed troops on the roof. They clashed with the attackers inside, killing two of them, arresting another four and declaring the standoff over.
At least four police were among those killed in the assault, which lasted around an hour and a half, according to the police and medical officials, who spoke on condition of anonymity because they were not authorized to brief reporters.
Following the attack, authorities shut down the city's highly fortified Green Zone, home to a number of foreign embassies and most of the country's political elite. A number of major roads, shopping malls and bridges around the Iraqi capital were also closed for fear of follow-up attacks.
Also on Monday evening, a car bomb in southeast Baghdad in a crowded market area killed five and wounded 12, according to hospital and police officials.
The Islamic State group often targets the country's Shiite majority with bombs in crowded marketplaces that are designed to kill large numbers of civilians.
The Islamic State group controls much of northern and western Iraq, but suffered a major defeat last month when Iraqi forces drove the extremists out of the western city of Ramadi, capital of the sprawling Anbar province.
The IS rampage across Iraq in the summer of 2014 was halted several miles away from Baghdad, but the extremist group has claimed a number of attacks in the heavily guarded capital since then.