Seventeen car bombs exploded in Iraq on Monday, killing at least 55 people in predominantly Shi'ite areas in some of the deadliest violence since Sunni insurgents including al Qaeda stepped up attacks this year.

Police and medical sources said the attacks, which appeared to be coordinated, were concentrated on towns and cities in Iraq's predominantly Shi'ite south, and districts of the capital where Shi'ites reside.

The car bomb attacks in busy streets and crowded markets underscore deteriorating security in Iraq, where nearly 4,000 people have been killed since the start of the year, according to violence monitoring group Iraq Body Count.