U.S. troops on boats in the Tigris river mistakenly killed six Iraqis Wednesday in an exchange of fire between the two sides north of Baghdad, Iraqi officials said.

The clash began when Iraqi troops at a checkpoint fired at approaching U.S. military boats near Tarmiyah, 30 miles north of Baghdad, police and security 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 near the site of the clash, the Iraqis said.

The U.S. military confirmed there was an incident of "mistaken fire" between U.S. and Iraqi forces while the U.S.-led coalition was conducting an operation in the area against suspected Al Qaeda in Iraq militants. A U.S. spokesman said aircraft were also involved but did not give 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 came 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 have joined the fight against Al Qaeda in Iraq and 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 tied to Saddam Hussein's regime or were involved in crimes.

About 100 Iraqis in the staunchly Sunni Baghdad neighborhood of Azamiyah protested Wednesday against what they called abusive tactics by the Iraqi government against members of awakening councils.

The demonstration highlights the growing distrust between the predominantly Shiite government and members of the mostly Sunni councils as the U.S. prepares to transfer those groups to Iraqi control.

Demonstrators 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, Jalal Eddin al-Sagheer, demanding that he return to his "country of origin which is Iran."

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.