Neighboring India warned that Tropical Cyclone Phet would cause heavy rains and gale-force winds along parts of its western coast.
Phet was centered in the Arabian Sea, 660 miles (1,060 kilometers) southwest of the Kutch area of Gujarat state, the India Meteorological Department said.
The storm was forecast to gain strength Friday and move closer to Oman before returning to Pakistan's southwestern coast, where it was expected to be felt Sunday.
In Karachi, the Maritime Security Agency dispatched speed boats to several dozen fishing boats that were at sea and oblivious of the warnings, spokesman Shakil Ahmed said.
Senior relief officer Munir Ahmed Memon said some 250,000 people in the districts of Thatta and Badin could be affected. He said hundreds of schools had been converted into relief camps.
Riaz Khan, Pakistan's chief meteorologist, said the cyclone would likely lose much of its strength by Sunday, but could still cause strong winds and heavy rain.
Thatta and Badin districts were the worst affected in 1992 when a cyclone killed 450 people and displaced some 200,000 others.
