At least three Lebanese Shia pilgrims were killed and seven wounded when a roadside bomb exploded near their bus in the Iraqi city of Ramadi, a police source said.

Wednesday’s attack in Ramadi, the capital of mainly-Sunni Anbar province and a former al-Qaeda stronghold, raised fears of renewed sectarian violence.

The bus, carrying 45 Lebanese pilgrims, was heading to Baghdad from Syria, the source said.

"A roadside bomb exploded near a bus carrying Lebanese pilgrims coming to Iraq, probably to visit the sacred shrines in Iraq," the source said, speaking on condition of anonymity.

Violence has eased in Iraq since the height of the war in 2006 to 2007 when tens of thousands were killed as Sunni and Shia fought one another.

But Sunni fighters linked to al-Qaeda remain a potent force, and often attack security forces and Shia targets in an attempt to stir up sectarian tensions.