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BAGHDAD, Sept 15 (Reuters) - A suicide car bomber killed eight people and wounded 15 near a police checkpoint in southwest Baghdad on Saturday, Iraqi police said.

The explosion took place near a market in the predominantly Shi'ite district of Amil in the capital, and all the dead were believed to be civilians, police said.

U.S. military commanders say increased security patrols in Iraq this year, backed by a "surge" of U.S. forces, has helped curb violence in Baghdad and regions around the city.

But the head of an al Qaeda-led network in Iraq said in an audiotape posted on Saturday that the group would launch a new phase of attacks to mark the Muslim holy month of Ramadan.

Abu Omar al-Baghdadi, leader of the self-styled Islamic State in Iraq, said the campaign would last until mid-October.

On Thursday a Sunni Arab sheikh, Abdul Sattar Abu Risha, who led a tribal alliance which helped push al Qaeda insurgents from much of Iraq's western Anbar province, was killed in a bomb attack outside his home.

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