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Thirteen people have reportedly been killed in a bomb attack on a Pakistan-based hospital which was receiving the Shia casualties from an earlier explosion.

The bomb ripped through the Jinnah hospital in the Pakistan's largest city, Karachi on Friday, injuring scores of people. "This happened in front of the emergency ward of," spokesman for the provincial government Jameel Soomro was quoted by the AFP as saying.

The blast occurred as the hospital was receiving the wounded from an earlier attack in which a bomber had targeted a bus packed with Shia mourners.

Twelve people died and dozens other were injured among the group of pilgrims marking the 40th day after the martyrdom anniversary of Imam Hussein (PBUH).

The hospital attack was followed by a second attack which was unsuccessful, Reuters reported.

Senior police officer Ghulam Nabi Memon said that law enforcement agents "successfully defused" a bomb in the premises of the hospital.

In December 2009, a deadly attack on an Ashura procession claimed the lives of nearly 50 people.

Over the past two years, some 3,000 people have died in bomb attacks and other militant operations throughout Pakistan.