US forces in Iraq have suffered some of their heaviest casualties in recent months, with 18 personnel killed.
US helicopters are a frequent sight in the skies over Iraq
In the worst incident, a US military helicopter crashed north-east of Baghdad killing all 13 people on board.
A website linked to the former ruling Baath party said an Islamic group shot down the aircraft, but there is no independent verification.
In a separate incident, the US military said five of its soldiers were killed in a clash with militants in Karbala.
The clash occurred at the provincial government headquarters in the Shia holy city, 110 km (70 miles) south of Baghdad.
It happened as thousands of pilgrims gathered in Karbala for the Shia mourning ceremony of Ashura, due to begin on Sunday.
The latest crash comes weeks after a US military helicopter crashed in western Anbar province, killing four troops.
A military statement said: "A US forces helicopter went down north-east of Baghdad this afternoon. Emergency Coalition Forces responded and secured the scene," Reuters news agency reported.
"Thirteen passengers and crew members were aboard the aircraft and all were killed," it said.
AFP news agency quoted Lt Col Josslyn L Aberle as saying there were nine passengers and four crew on board.
The BBC's Mike Wooldridge in Baghdad says a short while before the announcement, Medivac helicopters had been seen flying out of the city.
A Baathist website says a group calling itself Jaish al-Mujahideen [the Mujahideen army] says it shot down the helicopter.
Our correspondent says groups have often made such claims in similar situations in the past.
There have been previous US military helicopter crashes in Iraq but rarely causing casualties on a scale like this, our correspondent adds.
In 2005, 31 US troops died in a helicopter crash in bad weather near the Jordanian border, and in November 2003 two Black Hawks collided under fire over Mosul, killing 17 service personnel.