KARACHI, Pakistan—The Pakistani Taliban have tightened their grip over the country's commercial hub, officials and residents said, despite a five-month government crackdown here.

On Thursday, tentative peace talks with the government were thrown into disarray when the militants claimed responsibility for a roadside bombing that killed at least 12 police officers when the bus taking them to duty was destroyed near the city's southeastern Landhi neighborhood, an area the Taliban dominate.

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