(Adds details, eyewitness account)

FALLUJA, Iraq, Jan 24 (Reuters) - Militants detonated a pick-up truck full of explosives next to a police convoy as it drove northwest of Baghdad on Saturday, killing five policemen, including a lieutenant-colonel, police said.

Five other police officers and nine civilians were wounded in the bombing in Garma, 30 km (20 miles) from the capital, Police Colonel Ali Mishhan told Reuters television at the scene, where fireman hosed down the ash and rubble and a police bulldozer cleared away vehicles wrecked in the blast.

The attack in an area once dominated by al Qaeda and other Sunni Islamist militants took place a week before provincial elections seen as a key test of security gains after years of sectarian slaughter that followed the 2003 U.S.-led invasion.

"We were just driving along the road when we heard this massive explosion," said wounded policeman Mohammed Abdallah, as he sat on a hospital bed, his blue collar stained with blood, his sweater torn and his face caked in soot.

"We were tossed upwards by the blast. When I hit the ground, I realised that Lieutenant-Colonel Mohammed was dead."

Earlier reports said the attack was carried out by a suicide car bomber but police later said the vehicle that exploded was parked at the time of the blast.

Militants continue to stage attacks despite a sharp fall in overall violence in the last 18 months. U.S. officials have warned of a possible uptick in violence before the Jan. 31 vote.

Police say they believe al Qaeda cells are still active in Garma, in the vast western desert province of Anbar.

Anbar was the heartland of the Sunni Arab insurgency against U.S. forces until tribal chieftains turned on militants in 2006. The province is now relatively peaceful, but tension among Sunni groups have simmered ahead of next Saturday's vote. (Reporting by Fadhil al-Badrani; Writing by Tim Cocks and Michael Christie; Editing by Jon Boyle)

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