KARACHI, (Reuters) – A suspected suicide bomber on a  motorcycle killed 12 Shi’ites in Pakistan’s commercial capital  yesterday, followed hours later by a blast at a hospital where  the wounded were being treated which killed 13 people.

The violence is bound to raise further questions about the  effectiveness of security crackdowns on resilient al  Qaeda-backed Taliban militants at a time when Washington is  pushing Pakistan to help stabilise neighbouring Afghanistan.

In scenes that have become familiar in the state’s battle  with the Pakistani Taliban, the second blast sent a plume of  white smoke above Jinnah Hospital as distraught Pakistanis  transported dead relatives.

The bomb, which wounded 100, blew clothes and sandals off  bodies near ambulances. A teenage girl wept over what appeared  to be a female relative whose stomach was shredded by shrapnel.

Provincial Health Minister Saghir Ahmed said 15 people were  in critical condition.

“It (the explosion) happened right in the middle of  ambulances,” Reuters reporter Augustine Anthony said of the  second blast.

There were conflicting reports on the causes of the blasts,  with some police officials saying it was suicide bombers on  motorcycles and others saying the bombs were planted.

It’s not clear whether the attack was meant to trigger  sectarian violence or create the impression that the government  was incapable of stabilising nuclear-armed Pakistan.

Police said they defused a bomb in the premises of the  Karachi hospital treating the victims of the blast.

“The bomb was planted in a television set and we  successfully defused it,” said senior police officer Ghulam  Nabi Memon.

Pakistani Taliban have carried out waves of bombings at  crowded markets and army and police facilities, killing  hundreds of people since October in a bid to topple the  pro-American government of unpopular President Asif Ali  Zardari.