Iraqi security forces set up checkpoints in southeast Baghdad and sent out helicopter search parties on Monday after confirming three U.S. citizens had gone missing.
Media reported three U.S. contractors or trainers had been kidnapped with their translator in the Dora neighbourhood on Friday. But a senior police officer said authorities were still not certain what had caused their disappearance.
Two Iraqi army helicopters were seen hovering over Dora while police SUVs patrolled the streets, residents said.
The predominately Sunni Muslim district was a bastion of the insurgency against the 2003 U.S. invasion and the site of intense sectarian bloodletting that peaked around 2006-07.
Iranian-backed Shi'ite militias, seen as a bulwark in the fight against Islamic State, also have a presence in the area.
And the capital of Iraq, OPEC's second biggest oil exporter, has also seen a proliferation in recent years of well-armed criminal gangs that carry out contract killings, kidnappings and extortion.
Dubai-based news channel Al Arabiya said on Sunday that three U.S. nationals had been kidnapped with their translator. Citing its own sources, it said the Americans were "contractors or trainers" employed by U.S. companies at Baghdad airport.
The U.S. State Department said on Sunday it was working with Iraqi authorities to locate Americans reported missing, without confirming that they had been kidnapped.
"We have no clear indication about the circumstances of their disappearance," a senior police officer said on condition of anonymity.
The Iraqi government has struggled to rein in Shi'ite militias, many of which fought the U.S. military following the 2003 invasion and are accused of killing and abducting American nationals.
Iraq has witnessed a series of abductions of foreign nationals in recent months. At least 26 Qatari hunters kidnapped last month in the southern desert by unknown militants have not yet been found.
In September, 18 Turks taken in Baghdad by an armed group that used a familiar Shi'ite Muslim slogan were released following several weeks in detention.
The radical Sunni militants of Islamic State have maintained a limited presence in Baghdad, regularly claiming bomb attacks against Shi'ite neighbourhoods.