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As in similar attacks in recent months, the first, smaller bomb was detonated with the apparent intent to attract bystanders and police, after which the second, more deadly blast took place.

Police say a third explosive device was also found at the scene, but was safely defused by the bomb squad.

The Jamia Islamia madrasah is a Sunni religious school noted for its modern curriculum, which includes computer classes, and for its international student body.

Sectarian violence between extremists of the Sunni and Shia religious sects is not uncommon in Karachi.

But provincial government adviser Salahuddin Haider says the attack was probably not religious in nature.

Instead, he says, the culprits are likely members of international terror groups, such as al-Qaida, which he says hope to destabilize Pakistan by encouraging religious violence.

"It was an attempt to incite a particular religious group. I am happy that the leaders [of the madrasah] have given a statement saying that, 'We are being provoked,'" said Mr. Haider.

In recent weeks, Pakistan has arrested a string of alleged al-Qaida operatives, and has spent the past months hunting for other suspected foreign militants believed hiding in the country's remote tribal region.

Mr. Haider says terrorists frequently target Karachi because it is Pakistan's main economic hub, and the terror groups wish to scare away foreign investors in order to punish the country's leadership.

Another bomb blast on Saturday killed two people, although that incident is suspected to be the result of a business dispute.