"A US Marine Corps transport helicopter crashed on 26 January at approximately 1.20am (2220 GMT Tuesday) near al-Rutbah while conducting security and stabilisation operations," the military said in a statement on Wednesday.
The marines declined to provide the cause of the crash or to release any casualty figures.
It said a search and rescue operation was under way. The statement said casualties would be confirmed later.
Elsewhere, a car bomb exploded on the road to Baghdad's international airport, apparently targeting a US convoy, Iraqi police said.
The US military did not provide further information or details of casualties.
US armoured vehicles and
transports are attacked daily
In a related incident, the US military reported that one US soldier was killed and two others wounded in an attack north of Baghdad.
"One 1st Infantry Division Soldier was killed and two others wounded when anti-Iraqi forces attacked their combat patrol with rocket propelled grenades near Duluiyah," the statement said.
One of the wounded is in serious condition.
The town of Duluiyah - near Balad - is a region where US troops frequently come under fire and fighters are stepping up their attacks in the run-up to Sunday's elections.
Car bombers have frequently targeted US convoys on the road to the airport, considered one of Iraq's most dangerous highways.
In the southwest city of Kirkuk, two car bombs exploded on Wednesday, killing at least seven Iraqis, police said.
Police said one blast was near the city's police station and the other close to the central market area.
Armed groups have repeatedly targeted Iraqi police and soldiers with car bombs and ambushes in the run up to the country's elections on Sunday.
Government and election officials have also been targeted.
There are fears violence could
escalate on 30 January
On Tuesday, Kirkuk imposed a curfew and tough security in a bid to ward off expected attacks during the country's election scheduled for 30 January.
The previous night, a US Bradley fighting vehicle rolled into a canal during a patrol northeast of Baghdad, killing five soldiers from the army's 1st Infantry Division and injuring two others, the US military said.
The accident, which is under investigation, occurred near the town of Khan Bani Saad during fierce sandstorms on Monday night.
Another US soldier died of wounds from a roadside bomb that hit an American patrol in Baghdad, the military said.