KARACHI (Reuters) -- A suspected suicide bomber on a motorcycle killed 12 Shi'ites in Pakistan's commercial capital today, 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 stabilize neighboring 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 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, which 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 stabilizing 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.
Carnage in the home of Pakistan's stock exchange and main port could further discourage investors, who have watched the Taliban spread their violent campaign from strongholds in lawless areas near the Afghan border to major cities, including an attack on a mosque near the headquarters of the powerful military.
"A woman was calling me at the emergency ward, when the blast went off outside and then it was all darkness," said Seemi Jamali, a senior emergency room doctor recalling the explosion. "There was an absolute chaos and people ran out of hospital from whichever door they could access."
The violence shook Karachi, a city of around 18 million, as questions are being asked about the fate of Pakistan Taliban leader Hakimullah Meshud, who was wounded after a U.S. drone aircraft strike in the northwest of the country in December.
His appearance in a farewell video with the double agent suicide bomber who killed seven CIA agents in Afghanistan in December suggested Zardari's pro-American government now faces a more sophisticated enemy capable of striking outside the country.
The attacks had all the trademarks of the Taliban. Raja Umer Khattab, a senior police officer, said initial investigations suggested an Al-Qaeda-linked group was responsible.
Karachi has been largely free of Islamist violence over the past couple of years, but a bomb at a minority Shi'ite Muslim procession in late December that killed 32 people fuelled concern that the militants were expanding their fight to the city.
Karachi already has its share of problems. Dozens of political workers have been killed in tensions which raised questions about the future of Zardari's coalition.