Bulgaria's Defense Ministry said the helicopter was downed by missile fire and the victims included a three-member Bulgarian crew.
A Toronto-based charter company and Bulgaria's Transport Ministry said there were 11 people on board. The company said there were two bodyguards from Fiji on board, but the 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 the helicopter.
The six Americans worked for security contractor Blackwater USA, the U.S. Embassy said. The Moyock, N.C.-based contracting firm provides security for State Department officials in Iraq.
It was unclear whether the civilians' employers were under contract to the Pentagon or the State Department, U.S. officials in Washington said.
A Blackwater spokesman did not immediately return a call seeking comment Thursday.
Last year, four Blackwater employees were killed in Fallujah, 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 Marine assault on insurgents in the city.
The MI-8 helicopter went down about 12 miles north of Baghdad, the U.S. Embassy said. Video on television showed burning wreckage from the Russian-built craft and personal belongings scattered across a wide area.
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 said.
Greenaway said the six Americans were "doing some sort of security work." He said the victims included three Bulgarian crew members and two security guards from Fiji.
On March 13, 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 Hillah.
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