Attackers disguised as Iraqi police shot the three civilians after holding them up at a false check point near Hilla, 100 kilometres (60 miles) south of Baghdad, Polish military official Zdzislaw Gnatowski told AFP in Warsaw.

It is believed the US nationals were affiliated with the US-led Coalition Provisional Authority in Iraq.

Five suspects have been arrested over the killings, he said.

"The attackers, disguised as Iraqi policemen had set up a false checkpoint ... They stopped the car in which the two US civilian workers and their Iraqi interpreter were travelling at 8:00 pm (1700 GMT)," he said.

"They shot them on the spot," he said. "In their car, the soldiers discovered three bodies ... They also found arms."

"The soldiers arrested five men and have handed them over to the American police," Gnatowski said.

Poland heads a multinational force of 9,000 troops patrolling southern and central Iraq, with their headquarters in Hilla.

In northern Iraq, an off-target US mortar round killed one Iraqi civilian and wounded another when it struck a building in the village of Ejba, the US military said Wednesday.

"A mortar round from 3rd Brigade, 2nd Infantry Division accidentally struck a building in the southeast side of Ejba, killing one citizen and injuring another Monday," Taskforce Olympia, charged with the northern provinces of Nineveh, Dohuk and Sulaimaniyah, said in a statement.

"The coalition unit was conducting a mission against hostile forces in the area when one round fell short of its intended target. Soldiers were immediately dispatched to the scene."

The soldiers transported the casualties to a medical hospital for treatment and the incident is under investigation, the military said.

In nearby Mosul, a police chief narrowly avoided an attempt on his life.

"Three unknown persons in a car opened fire on chief Abdel Azal Haffudi in the city centre. In the ensuing exchange of fire, one assailant was killed and two others injured," police Colonel Mohammad Abdallah Obeidi told AFP.

In other developments, a bomb exploded early Tuesday some 50 kilometres (30 miles) northwest of Baghdad, near the Baquba office of the Supreme Council for the Islamic Revolution in Iraq (SCIRI), one of the main Shiite parties.

"Around 6:00 am (0300 GMT), a bomb placed by the Badr Organisation office exploded left four people wounded, including the four year-old son of one of the security guards," said Faraj Hatem, a SCIRI official.