Gunmen in military uniform seized 18 Turkish workers from a sports stadium they were building in Baghdad on Wednesday.
Turkey said it appeared to have been a targeted attack. Diplomats have said Turkey could suffer reprisals after abandoning months of reticence to launch airstrikes against ISIS in neighboring Syria and open its bases to a U.S.-led coalition fighting the Sunni Muslim militants.
The workers were taken in the predominantly Shiite district of Habibiya, Iraqi Interior Ministry spokesman Brigadier General Saad Maan said.
There was no immediate claim of responsibility.
"People dressed in military uniforms broke down the door at 3 a.m. and abducted all these people," said Ugur Dogan, chief executive of Nurol Holding, which owns the construction firm.
ISIS regularly claims responsibility for suicide bomb attacks in Baghdad, but Shiite militias and other armed groups are also active there.
The Shiite-led government in Baghdad has intensified security in the city this week ahead of plans to ease access to the fortified Green Zone and eliminate no-go zones set up by militias and political parties.
Two police sources said the gunmen had arrived in a convoy of black sports utility vehicles. Maan denied reports that up to three Iraqis had also been abducted, and put the number of people kidnapped at 16.
A spokesman for the Turkish foreign ministry said Turkish nationals had been specifically picked out by the attackers, without giving further details.
An ISIS video released last month accused Turkish President Tayyip Erdogan of "selling the country to crusaders" and of allowing U.S. access to Turkish bases "just to keep his post."