A US military spokesman said the cause of Saturday night's crash of the Black Hawk near the city of Tal Afar in northwestern Iraq was under investigation
He said that there was severe weather at the time the helicopter went down.
"A coalition helicopter crashed in a sparsely populated area 12 kilometres east of Tal Afar shortly before midnight (2100 GMT), killing passengers and crew," a US military statement said.
Communications lost
The Black Hawk was part of a two-aircraft mission flying between bases in northern Iraq when communications were lost with the aircraft on Saturday night.
"Flight records indicate that eight passengers and four crew members were manifested for the flight. An immediate search and rescue operation was launched from nearby military installations."
The spokesman said he could not immediately give details about those on board the plane which was found at about noon (0900 GMT) on Sunday.
It was the second deadliest US military crash in Iraq since the March 2003 invasion, after a transport helicopter went down last January, killing 31 aboard.
Earlier, the US military announced that five US marines had been killed in and around the restive western Iraqi town of Fallujah over the weekend.
Three "were killed by small-arms fire in separate attacks while conducting combat operations against the enemy" on Sunday.
Another two were killed by roadside bomb explosions to the south and east of the city on Saturday.
The latest casualties took the death toll of US soldiers in Iraq since the March 2003 invasion to over 2,200.
Hostage escapes
In a rare glimmer of hope in the hostage crisis blighting Iraq, a French engineer kidnapped by insurgents five weeks ago in Baghdad has escaped and even tried to help US troops arrest his captors.
Bernard Planche, 52, on Saturday ran away from a farmhouse where he was held after his captors fled US and Iraqi troops who were conducting a search of a rural area on the western outskirts of the capital.
The former hostage then insisted on staying with US troops for six hours to help them hunt for his former captors, the US military said, without giving details of any arrests.
France's Foreign Minister Philippe Douste-Blazy said Mr Planche was taken on Sunday afternoon to the French embassy in Baghdad from where he was to return to France shortly.
The engineer, who was working on water projects, is the fourth French hostage to survive a kidnapping in Iraq since the US-led invasion.
He was seized by an Iraqi rebel group on December 5 from his home in an upmarket Baghdad neighbourhood.
The group, calling itself the Battalion of the Lookout for Iraq, had threatened to kill him if France did not "end its illegitimate presence in Iraq" in a video broadcast by Al-Arabiya television on December 28.
The French government pointed out it has no military presence in Iraq.
Kidnap video
The Dubai-based channel showed a black-and-white video bearing the group's logo and showing a man with a moustache, on his knees, with two gunmen standing behind him.
Two days later, the man's brother and daughter appealed for his release on Al-Arabiya, saying he was in Iraq only to help the Iraqi people.
Hostage-taking has become commonplace in the country over the past two years, as Westerners pay the price for their governments attracting the wrath of rebel groups that opposed the US-led war and its foreign occupation.
Several Westerners are still being held hostage in the country, including a number of Americans, a Briton and two Canadians. Four members of a Christian peace group were kidnapped in November in Baghdad.
Zarqawi tape
Meanwhile the leader of al-Qaeda in Iraq, Abu Mussab al-Zarqawi, has denounced Arab countries working for political reconciliation in Iraq as US agents, according to an audio tape posted on the Internet.
"The countries that met in Cairo ... were involved in destroying Iraq and cooperated with America by opening their land, air space and waters and offering intelligence to it," said the speaker on the tape, who sounded like Zarqawi.
He was referring to an Arab League conference in November that tried to reconcile Iraqi political factions.
The tape, posted on an Islamist website often used by insurgent groups in Iraq, could not be authenticated.
The speaker denounced the Iraqi Islamic Party, viewed as the largest Sunni Arab party, for endorsing a new constitution, a move which boosted the Shi'ite- and Kurdish-led government.
"We call on the Islamic Party to leave this path ... which leads to the destruction of the Sunnis," the speaker said.
"We had the power to disrupt the elections in most parts of Iraq but did not do it in order not to harm the Sunni masses," the speaker said, referring to last month's parliamentary polls which were mostly peaceful.
Osama bin Laden named Zarqawi as his deputy in Iraq after he pledged allegiance to the overall al Qaeda leader in October 2004. Washington has set a $25 million US bounty on Zarqawi's head.