The Russian-built Mi-8 helicopter was shot down as it flew over a deserted area north of Baghdad, Bulgarian officials and the US military said on Thursday.
The Bulgarian Defence Ministry said the helicopter was struck by missile fire, but US State Department spokesman Adam Ereli said he could not confirm the cause of the crash.
It was thought to be the first downing of a civilian aircraft in Iraq.
The Bulgarian company that owned the downed helicopter said that besides three Bulgarian crew, there were six American passengers and two guards from an unspecified country on board.
A group campaigning against the US presence in Iraq claimed responsibility for the attack on the helicopter, according to an internet statement.
"The Islamic Army in Iraq claims responsibility for bringing down a ... cargo aircraft and killing all those on board," said the short posting on a website, adding that a fuller statement and a video film of the attack would follow.
"One of the crew members was captured alive and killed," it said.
The group said the crew member was killed to "avenge Muslims killed in cold blood in Falluja's mosques ... in front of the eyes of the world and on television screens without anyone protesting".
An Mi-8 helicopter like this one
crashed on Thursday
All on board were civilians, US military officials said. The Americans worked for Blackwater Security Consulting.
A Canada-based charter company said the two additional passengers were bodyguards from Fiji, but the Bulgarian Transport Ministry said they were from the Philippines.
The Philippine mission in Baghdad said it had no information that any of its nationals were on board the helicopter.
Two American military officials in Baghdad initially said the helicopter was leased by the US Defence Department, but the US embassy later said that was untrue. It gave no further information.
It was unclear whether the civilians' employers were under contract to the Pentagon or the US State Department, US
officials in Washington said.
Last year, four Blackwater employees were killed in Falluja, and their bodies were burned and mutilated. Two of the corpses were strung up on a bridge over the Euphrates river. The deaths touched off a US marines assault on the city.
The Mi-8 helicopter went down about 20km north of Baghdad, the US embassy said. Video on television showed burning wreckage from the craft and personal belongings scattered across a wide area.
The Bulgarian Defence Ministry issued a statement saying the helicopter was struck by missile fire. US officials said they
presumed the helicopter was shot down but could not confirm that until an investigation was completed.
The helicopter was owned by Bulgaria-based Heli Air and chartered by Toronto-based SkyLink Aviation Inc, said SkyLink air operations manager Paul Greenaway. The helicopter was flying to Tikrit, he added.
Greenaway said the six Americans were "doing some sort of security work".
On 13 March, two American security contractors working for Blackwater Security, a subsidiary of Blackwater USA, were killed and a third was wounded in a roadside bombing south of Baghdad on the main road to Hilla.
Meanwhile, in Ramadi, 115km west of Baghdad, a roadside bomb wounded one soldier in a US convoy. When another US soldier fired his machine gun at a suspected Iraqi ambush site, a female Iraqi civilian was wounded, then died in a hospital, US officials said.
Car bombings have been used
frequently by anti-US fighters
Hours later, gunfire erupted downtown, and an Associated Press photographer at the scene saw a young boy lying dead in a street near three smouldering cars. Sporadic gunfire continued for about two hours, the photographer, Bilal Husain, said.
When it subsided, Iraqis pulled the charred body of an adult from one of the burned cars, he said.
It was not clear how the two had been killed.
The US military had no immediate information on the second incident.
In related news, the Iraqi Defence Ministry identified 19 bullet-riddled bodies of fishermen, not soldiers as initially rumoured -found on Wednesday in Haditha, 220km northwest of Baghdad.
Investigations indicated that the men had come from the southern Diwaniya and Najaf provinces to fish in Tharthar lake when they were captured by fighters, taken to the football stadium at nearby Haditha and shot, said Salih Sarhan, the ministry's chief spokesman.
He did not say how the victims had been identified or why they might have been captured.
Also on Thursday, a bomb targeting Western workers in Iraq exploded, killing a security contractor.
A spokeswoman for the British-based Aegis defence services said one employee was killed and another injured as their car headed from the capital towards the airport.
"Aegis Defence Services can confirm that one member of staff was killed in the line of duty in Baghdad today and another person sustained injuries," she said. She gave no details as to their identities.
This Iraqi civilian was wounded by
the Airport Road bomb
Aljazeera reported the two were US citizens.
One Iraqi civilian, travelling in another car, was also hurt by the blast, according to an Interior Ministry official.
Aljazeera also reported an explosion caused by a car bomb targeting a US convoy near al-Amil neighbourhood.
Iraqi journalist Muhammad Abd Allah told Aljazeera an Iraqi driver was also wounded in the attack and was transferred to a nearby hospital.
No other casualties were immediately reported, but one of the vehicles was consumed by flames, and white smoke rose from another, police Captain Hamid Ali said.
All three vehicles appeared to have been blown off the road by the impact of the explosion, witnesses said.
The road to the airport is only a few kilometres long and flanked by US military bases.
Abd Allah said al-Matar (Airport) Road was known as "Death Road".