KARACHI, Pakistan—Large neighborhoods in Pakistan's biggest metropolis have fallen under the control of the Taliban over the past year, according to residents and officials here, a development that could disrupt the country's economy and endanger the U.S. military pullout from Afghanistan.

The Pakistani Taliban, a movement that was spawned by the Afghan Taliban but now operates separately, are now the dominant force in entire districts of this sprawling city of 20 million people, residents said. The group has established tribal...