Meanwhile, about 100 Iraqis in a staunchly Sunni Arab neighborhood of Baghdad protested what they called abusive tactics by the Iraqi government against members of U.S.-backed groups that have joined the fight against al Qaeda in Iraq.
The demonstration highlights the growing distrust between the predominantly Shiite government and members of the mostly Sunni "awakening councils" as the U.S. prepares to transfer those groups to Iraqi control.
Wednesday's early morning shootout began when Iraqi troops at a checkpoint fired at approaching U.S. military boats near Tarmiyah, 30 miles north of Baghdad, Iraqi officials said. They did not realize the boats, which had their lights off, were American.
The U.S. soldiers fired back, killing two Iraqi soldiers, two police officers and two U.S.-backed Sunni tribesmen, said the officials, who spoke on condition of anonymity because they were not allowed to release the information to the media.
Two U.S. helicopters later fired on a one-room house on an island in the river, the Iraqis added.
The U.S. military confirmed that coalition forces were conducting an operation in the area against suspected al Qaeda in Iraq militants when an "incident involving weapons fire" occurred between the U.S. soldiers and Iraqi security forces.
A U.S. spokesman said aircraft was involved but didn't immediately release more details.
"It is always regretful when incidents of mistaken fire occur on the battlefield," the spokesman, Maj. John Hall, said in an e-mail. An investigation was under way, he said.
The shootout happened two days after a suicide bomber attacked the home of a local awakening council leader in Tarmiyah. The leader was wounded and another member of the U.S.-allied Sunni group was killed, the U.S. military has said.
The awakening councils are one of the key reasons why violence in Iraq is at its lowest levels in years.
But there is concern that those armed Sunni groups could turn their guns on the Shiite-led government if it fails to deliver on promises of economic help, jobs in the security forces and a share of political power.
Recently, the government has moved against some awakening groups in Diyala province, arresting some of their leaders and evicting them from offices. Government officials complain that many council members had ties to Saddam Hussein's regime or were involved in crimes.
In the Baghdad neighborhood of Azamiyah, demonstrators on Wednesday held signs protesting what they say is unfair government policies against Sunnis. They also warned the national leadership against close ties with Shiite-dominated Iran, which many Sunnis believe is backing Shiite extremists in this country.
Members of crowd shouted slogans against a high-profile Iraqi Shiite cleric, demanding that he return to his "country of origin which is Iran."
Those sentiments are likely to sharpen as the government assumes greater control over armed Sunni groups. Beginning next month, the Iraqi government will assume control over the contracts of 54,000 awakening council members in Baghdad province, the U.S. military said this week.
As part of the handover, the central government will begin paying the members Nov. 1. The U.S. military currently is in charge or funding the awakening councils.