Kidnappings are a major problem in Baghdad and other parts of the country and most frequently target Iraqis, but Qatari and Turkish citizens have also been seized in recent months.
"We are aware of reports that American citizens are missing in Iraq," US State Department spokesman John Kirby said.
"We are working with the full cooperation of the Iraqi authorities to locate and recover the individuals," Kirby added, without providing details about their number or the circumstances of their disappearance.
The officer said that according to information he had received, the kidnappers were militiamen wearing military uniforms.
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"We don't know what their work is," the colonel said of the kidnapped Americans.
Iraq turned to paramilitary forces dominated by Iran-backed Shiite militias to help combat the Islamic State jihadist group, which overran large parts of the country in 2014.
These groups, which fall under an umbrella organisation known as the Hashed al-Shaabi, or Popular Mobilisation units, have played a key role in the fight against the jihadists.
The US is leading a coalition of countries that have bombed thousands of IS targets in Iraq and Syria and which are providing training to Baghdad's forces.
Washington has also dispatched special forces to Iraq to carry out raids against the jihadists.
Both American forces and Shiite paramilitaries are battling IS, but relations between the two sides have been tense, especially due to fighting between them in the years after the 2003 US-led invasion of Iraq.
IS also has ample motive to target Americans, but while it is able to carry out bombings in Baghdad, it does not have a major presence in the city.
Last month, gunmen kidnapped more than two dozen Qataris who had come to southern Iraq to hunt.