Suspected Sunni militants bombed a bus carrying Shiite worshippers and two hours later attacked a hospital treating the victims, killing 25 people and wounded 100 on Friday in a strike on Pakistan's largest city.

The blasts in the southern city of Karahi were the latest sign of the instability tearing at the nuclear-armed nation, which the United States regards as key to its hopes of defeating a related Taliban insurgency across the border in Afghanistan.

Prime Minister Yousuf Raza Gilani appealed for calm in the city, which is the country's commercial heart. It has a history of religious violence between Shiite and Sunni Muslims, and has been tense in recent weeks due to clashes between rival political parties that have left dozens dead.

No group claimed responsibility, but Pakistan is home to many Al Qaeda-linked Sunni extremist groups with a history of attacking Shiites.

In late December, extremists in the city detonated a bomb that killed 44 Shiites attending a procession to mark Ashura, the anniversary of the death of revered Shiite figure Imam Hussein, sparking the city's worst riots in recent years. Friday's blasts coincided with Arbaeen, the final day of the annual 40-day mourning period for Hussein.

Shiites marking Arbaeen were also attacked in Iraq on Friday. A suicide attacker detonated a car bomb alongside a crowd of pilgrims walking to the holy city of Karbala, killing at least 27 people. It was the third deadly bombing this week against Shiites.

Police officers in Karachi gave conflicting accounts whether one or both of the bombs there were suicide blasts. Both were apparently attached to motorbikes and were packed with nuts and bolts, an investigator said.

The first bomb targeted a bus carrying worshippers, most of them women and children, killing 12 and wounding 49, officials said. The bomb was attached to a motorcycle and detonated as the bus drove to an Arbaeen procession, witnesses said. One witness said it may have been a suicide bombing, but that could not be confirmed.

The second bomb exploded outside the entrance to the emergency ward at Jinnah Hospital, which was packed with victims and relatives of those killed and wounded in the earlier attack. It was either hidden on a motorbike or in or close to an ambulance, a witness and a government official said.

Provincial health minister Dr. Sagheer Ahmad said 13 people were killed in that blast and 50 wounded.

The blast triggered chaos at a hospital already overrun with casualties. A man held up a child bleeding from his face above his head in a bid to pass through the crowd and get treatment inside. Another victim with a chest wound lay on a stretcher crying for help.

Bomb disposal squad official Munir Sheikh said a third bomb was later defused in the parking lot of the hospital.

Pakistan has been rocked by a wave of bombings over the last 18 months by the Pakistani Taliban, which is under pressure by U.S. missile strikes and army offensives in its stronghold along the border with Afghanistan, far from Karachi.

While sharing the anti-Shiite agenda of other Sunni militants, the Taliban has concentrated mostly on attacking government, military and Western targets. Karachi has been spared those attacks. Some officials have said this is because the Taliban use the city to raise funds through extortion and other criminal activities and do not wish to draw attention to their presence.

The bombings brought fresh tales of grief in a country where such attacks have almost become routine.

Ashfaq Ali survived the bus attack, but lost two sons. He sat on the floor near a pool of blood.

"I will keep sitting here because it is my sons' blood," he said, half-wailing. "I want the terrorists to kill me as well."

Friday's attack could exacerbate political tensions in the city of 16 million people. Accusations the city's rulers failed to prevent the December attack contributed to the near breakdown between the two main parties that make up its fragile coalition.

Pakistan's Sunnis and minority Shiites generally live in peace, but attacks, mostly on Shiites, have often occurred over the last 20 years. Al Qaeda, the Taliban and other Sunni extremist groups despise Shiites, believing them to be infidels.