BAGHDAD, May 9 (Reuters) - A U.S. attack helicopter killed five bystanders including two children when it fired on insurgents north of Baghdad, the U.S. military said on Wednesday.

Lieutenant-Colonel Mike Donnelly, a spokesman for U.S. forces stationed north of the Iraqi capital, denied some media reports that the helicopter had fired on a school on Tuesday.

"It's traumatic and entirely unfortunate that this happened," Donnelly said by telephone, adding an investigation had been opened into how the civilans could have been killed.

He said insurgents had been seen placing roadside bombs and operating an illegal checkpoint near the town of Mandali in Diyala province, prompting the U.S. military to call for air support.

The helicopter attack killed two of the insurgents, but interviews with residents later showed that the five civilians had also been killed, Donnelly said.

He had no information on the age of the children.

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