A suicide bomber disguised as a policeman killed at least 15 people in an attack that targeted Shi'ite Muslim pilgrims passing through a checkpoint in Iraq's southern city of Basra on Saturday, police said.

The attack at the end of Arbain, one of the main holy days of the Shi'ite calendar, occurred at a time when a political crisis in Iraq's Shi'ite-led government has renewed fears of a return to sectarian violence in the country.

A terrorist wearing a police uniform and carrying fake police I.D. managed to reach a police checkpoint and blew himself up among police and pilgrims, said a police official at the scene of the bombing.

At least 15 were killed, including 11 policemen, and another 40 people were wounded when the bomber detonated his explosives among pilgrims trying to pass through the checkpoint on their way to a major Shi'ite mosque to the west of Basra.

Arbain has been a frequent target of militants since the U.S.-led invasion that ousted Sunni dictator Saddam Hussein.

Scores of people have been killed in attacks on pilgrims in the last few weeks, including a suicide bombing which killed at least 44 people.

Many of the attacks on pilgrims have used methods such as suicide bombings that are the signature of Iraq's al Qaeda affiliate.

Attacks targeting Shi'ites have also killed dozens of people since Shi'ite Prime Minister Nuri al-Maliki's government issued an arrest warrant for a Sunni vice president, triggering a political crisis that risks scuttling a power-sharing agreement.

Attacks had ebbed since the height of sectarian slaughter in 2006-2007 when thousands were killed in intercommunal violence among Shi'ite and Sunni. But the withdrawal of the last American troops in December has fanned worries of a spike in violence.

(Additional reporting by Ahmed Rasheed, Writing by Patrick Markey, Editing by Matthew Jones)