U.S. forces in Iraq, already implicated in the killing of an Italian secret agent, faced further strains with allies on Monday when Bulgaria said they had probably shot dead one of its soldiers.
The controversy raised questions over whether U.S. troops are over-zealous in their efforts to crack down on insurgents, who killed at least 19 people in fresh attacks on Monday.
The Bulgarian soldier was killed in southern Iraq on Friday evening, around the same time that U.S. forces in Baghdad opened fire on a vehicle taking kidnapped Italian journalist Giuliana Sgrena to the airport shortly after her captors freed her.
Sgrena was wounded in the shoulder and secret agent Nicola Calipari, who played a key role in her release, was killed. Hundreds of Italians lined the streets of Rome on Monday for the state funeral of Calipari, who died attempting to shield Sgrena.
Bulgarian Defence Minister Nikolai Svinarov said an investigation into the death of the Bulgarian soldier showed he was probably accidentally killed by American troops.
"Someone started shooting at our patrol from the west, and in the same direction, 150 metres (yards) away, there was a unit from the U.S. army," he told a news conference.
"The result gives us enough grounds to believe the death of rifleman Gurdi Gurdev was caused by friendly fire."
Svinarov said the Bulgarian army's chief of staff had written to General Richard Myers, chairman of the U.S. joint chiefs of staff, asking for an investigation.
The U.S. military had no immediate comment.
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