They said the 18 are employed by Nurol Insaat, a Turkish construction company contracted to build a sports complex in the sprawling Shiite district of Sadr City. The kidnappers stormed the construction site, where the workers were sleeping in caravans, breaking down doors and disarming the guards before taking the workers away.
The Iraqi officials said an Iraqi national was kidnapped along with the Turks.
Turkish Foreign Ministry Spokesman Tanju Bilgic said those kidnapped included 14 workers, three engineers and one accountant. He said the kidnappers specifically targeted Turkish nationals, picking them out from the rest and leaving behind workers from other countries.
Turkish Deputy Prime Minister Numan Kurtulmus confirmed the kidnapping and said Ankara was in close contact with authorities in Iraq. "The Iraqi authorities for the time being do not have information on how the incident occurred or who captured them," he told reporters.
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In Baghdad, Interior Ministry spokesman Saad Maan told The Associated Press that authorities are investigating the incident and that a contingent of security forces has been tasked with tracking down the kidnappers.
Neither the identity nor the motives of the kidnappers were immediately known.
The Iraqi officials spoke on condition of anonymity because they were not authorised to speak to the media.
Turkey recently began launching airstrikes against the Islamic State group in Syria and allowing US warplanes to use bases in southeastern Turkey to strike the Sunni extremist group.
It launched a simultaneous air campaign in northern Iraq against the Kurdistan Workers' Party, or PKK, a Kurdish militant group.
IS militants seized 49 diplomatic staff and family members from the Turkish consulate in Mosul when they captured the northern Iraqi city in June 2014. The group held them for three months before releasing them unharmed. Turkish officials have suggested - but never formally confirmed - that the release was secured in exchange for IS prisoners held in Turkey.