A bomb attack in southern Iraq has killed at least 20 people. The blast struck a group of Shi'ite pilgrim near the city of Basra.

Police and medics say it is not clear whether the blast Saturday came from a suicide bomber or a roadside bomb.

The attack took place as Shi'ites journeyed to a shrine near Basra to observe the end of an annual period of mourning.

The attack was the latest in a series against Shi'ites during their religious commemorations. Tensions between Shi'ite and Sunni Muslims in Iraq are high, especially since the withdrawal of U.S. troops from the country late last year.