Unknown gunmen shot dead three policemen in western Baghdad on Monday, an Interior Ministry source said.

"Unidentified armed men opened fire at a police vehicle near the Bilal mosque in Yarmouk district at around 10:00 AM (06:00 GMT), killing the three policemen on board," the source told Xinhua on condition of anonymity.

Earlier, a ministry source said that a roadside bomb ripped through a bus on Monday, killing 11 construction workers and wounding 16 others.

The attack occurred at about 7:45 AM (03:45 GMT) on the main road near Khalis, 80 km north of Baghdad, the source said.

A day before, a tribal chief died when gunmen showered his car with bullets in Baghdad's western district of Mansur, a police source said.

Sheik Osama al-Jadaan, his driver and one of his bodyguards were killed in the attack, the police said.

Al-Jadaan agreed to help Iraqi security forces crack down on the al-Qaida members and foreign fighters in western Anbar province where his Karabila tribe resides.

Anbar is also insurgent bastion stretching from Baghdad to the Syrian border.

(Xinhua News Agency May 29, 2006)