Updated at 7:40 a.m. Eastern.
A bomb tore through a bus carrying Shiite Muslim worshippers in the southern Pakistani city of Karachi on Friday, killing at least 12 people, officials said. A second blast, apparently targeting family members from the first explosion, struck outside a hospital in Karachi shortly after the initial attack.
Karachi has seen a number of attacks aimed at minority Shiites in recent weeks, including one that killed dozens at a procession for a religious holiday and sparked subsequent riots. Extremists from Pakistan's Sunni Muslim majority have been blamed.
Police chief Waseem Ahmad appealed to Shiites in the teeming, chaotic city of more than 16 million to remain calm.
The first bomb apparently was attached to a motorcycle, detonating as the bus drove by.
A doctor said at least five people were killed in the hospital attack, which CBS News' Maria Usman reports occurred amid a group of ambulances outside the Jinnah hospital, where victims of the first explosion were being treated.
Local TV footage showed security officials examining the twisted metal pieces of the wrecked motorbike. The bus nearby appeared severely damaged. Windows in nearby buildings also shattered.
The bus carried mainly women and children headed to a Shiite religious gathering, senior police official Javed Akbar said.
Dr. Simi Jamali said at least 11 bodies were brought to a local hospital.
Pakistan's Sunnis and Shiites generally live in peace, but extremists from the two sects have targeted one another's leaders and worshippers.
Karachi is a port city and the main commercial hub in Pakistan. It has a history of political, ethnic and religious violence. The Taliban, a Sunni Islamist movement, also are believed to use Karachi as a place to rest and raise money.