The US embassy in Baghdad has confirmed that "several" Americans have been kidnapped there.
"We are working with the full co-operation of the Iraqi authorities to locate and recover the individuals," said a state department official.
Unconfirmed reports from Iraqi security circles said three Americans and an Iraqi translator were seized on the southern side of Baghdad.
But US officials did not say how many were held or where they were seized.
An official in Baghdad told CNN three contractors went missing on Friday.
"A company filed a report Sunday about three of its staff going missing two days ago. They are American contractors. We are looking into this report," the senior security official said.
"The safety and security of American citizens overseas is our highest priority," said US state department official John Kirby.
If it turns out to be a serious kidnapping, reports the BBC's Jim Muir in Baghdad, it would be a major blow to hopes for stability and progress in Iraq.
An attack on a Baghdad shopping centre last week, claimed by so-called Islamic State militants, brought to an end a relative lull in violence that had seen no major bloodshed in the city in months.
Before US forces pulled out of Iraq in 2011, a number of Western citizens were kidnapped and killed by radical Shia groups as well as Sunni militants, but none has been abducted since then, our correspondent says.
Last month, a group of Qatari hunters, including members of the ruling family, was kidnapped in the Iraqi desert.
Since then, BBC Iraq correspondent Ahmed Maher says, there has been no claim of responsibility and the fate of those kidnapped remains unknown - a sign of how secretive and complicated the negotiation process is.