One witness said on Monday US troops surrounded the crash site and that smoke was rising from the area near the small town of Mishahda.
The US military confirmed the crash and said there was no immediate news of its two-man crew.
"A task force Ironside helicopter did go down this morning at 8.20am (0520 GMT), a spokesman said.
The cause of the accident, a few kilometres north of the capital, is under investigation, he added.
On Friday, two US pilots were killed when their reconnaissance helicopter was apparently shot down in northern Iraq, while eight US soldiers and four civilians died a week earlier when their Black Hawk helicopter crashed near the town of Tal Afar in northwestern Iraq.
An internet video purportedly of a missile shooting down a US helicopter near Baghdad on Monday was posted on a website usually used by Iraqi fighters opposed to US military presence.
Monday's helicopter crash was the third that the US army in Iraq has lost in one week.
Coast guard clash
In other news, the Iranian coast guard killed an Iraqi soldier and kidnapped nine others in a confrontation with the Iraqi coast guard, the governor of Basra told Aljazeera on Monday.
Mohammed al-Waili said the incident occurred on Saturday after Iraqi forces tried to stop a ship from smuggling oil out of Iraq.
"When they saw our boats coming closer to the ship ... the captain of this ship - who is Iranian - was able to call Iranian forces ... and Iranian boats then opened fire on our boats," he said.
Basra's governor says Iranians
are holding two Iraqi boats
"This is not the first time that Iranian forces have attacked our patrols," he said.
Aljazeera also quoted Waili as saying Iranian forces were holding two Iraqi boats in Abbadan port.
The governor complained about Baghdad's apathy, saying he has brought the Iranian attacks to the notice of the federal authorities but has received no response yet.
Iraqi police said on Monday they had found seven unidentified bodies in different parts of central and southern Baghdad.
In other news, six Iraqi people, including five policemen, were killed in a car bomb blast targeting a convoy of police vehicles near the city of Muqdadiya, north of Baghdad.
An official at the city's hospital said that another 16 people were injured in the blast.
Elsewhere, a policeman was killed by an armed man on his way to work in the capital's Aadhamiya district.
In Baladiyat district, six members of the Iraqi National Guard were injured and two of their vehicles destroyed when a bomb aimed at their patrol exploded, Iraqi security forces said.
In central Baghdad, a bomb exploded near a convoy of civilian vehicles, triggering a shoot-out between Iraqi police and fighters, but nobody was hurt in the violence, a police officer said.
Iraqi and British soldiers arrest
suspected fighters in the south
The explosion was the first in the city centre after a weeklong Muslim holiday lull.
Four four-wheel drive cars, frequently used by foreign security contractors and diplomats to move around Baghdad, were targeted in the attack, the police officer on the scene told AFP, speaking on condition of anonymity.
It was not immediately clear who had been travelling in the convoy, which appeared to have been escorted by two overhead helicopters.
The bomb only caused minimal damage to one of the vehicles in the convoy, the officer said.
Earlier, an Interior Ministry official said US and Iraqi forces had been involved in the shooting. A US military spokesman said he was looking into the report of the explosion.