A Taliban suicide bomber assassinated a top-ranking Pakistani security official and key U.S. ally Wednesday, adding to a string of crises here that have raised alarm in recent days over whether the government can cope.

Severe flooding across the northwest, in particular, threatens to take Pakistani leaders' attention away from efforts to eliminate key al-Qaeda and Taliban sanctuaries.

Pakistani officials insist that they are doing their best with limited resources to assist flood victims and that their efforts will not detract from the fight against extremist groups. But U.S. officials say they are concerned that the flood could become a major internal catastrophe if more is not done to help the victims.

With the government already facing political turmoil, such a crisis could be destabilizing for Pakistan, officials say, and would eliminate all hope of persuading the nation's military to step up efforts against insurgent groups that use Pakistan as a base from which to attack U.S. troops in Afghanistan.

Since last week, Pakistan has endured its worst air crash, its most devastating floods in living memory, its deadliest ethnic riots this year and now the killing of a commander known for his vigor in hunting insurgents.

While not all connected, the confluence of so many disasters -- both natural and man-made -- has appeared to overwhelm a government that has trouble performing basic services even during the best of times.

"It is all too much," said Javed Hussain, a security analyst and retired Pakistani general. "The problems are big, but the leaders are small."

Pakistan's northwest, where the flooding has been concentrated, is also the area where militant groups are most active. With at least 1,500 people dead and 3 million affected by the flooding, the vast scale of destruction has created needs that the government admits it is unable to meet. Anger has been rising all week in the northwest, where residents say they have seen little sign of a coordinated assistance program.

The United States has earned rare praise here for reacting swiftly to the floods, promising $10 million in aid, flying in six helicopters from Afghanistan and providing hundreds of thousands of ready-to-eat meals.

The response reflects U.S. recognition of both the peril if humanitarian needs continue to go unmet and the promise of a chance to rehabilitate the American brand in a country where U.S. policies are unpopular.

The Taliban may see opportunity in the floods. In the void left by the government, charities that are known fronts for Islamist militant groups have taken to the streets to distribute food, medicine and tents.

Analysts say, meanwhile, that Wednesday's suicide bombing may be the first in a series of attacks as the armed forces turn their focus to flood relief.

"The pressure on the Pakistani Taliban has been considerably released," Hussain said. "They are ruthless people, and they are going to exploit this."

Wednesday's attack came in one of the most heavily patrolled areas in the regional capital of Peshawar, outside the office of the Frontier Constabulary. The constabulatory's chief, Sifwat Ghayur, had just stepped into his car when a suicide bomber walked up to the vehicle and blew himself up, police said.

The Frontier Constabulary, along with the Frontier Corps, forms a central front for Pakistani security in the tribal lands along the border with Afghanistan. Unlike the Pakistani army, the Frontier forces are made up almost exclusively of ethnic Pashtuns native to the region.

The United States has bet heavily on efforts to professionalize the forces, pouring money into training and equipment programs in the hope that the Pashtun troops will take a more active role in confronting the Taliban and al-Qaeda.

Ghayur had been Peshawar's top police official before he was promoted to run the Frontier Constabulary, and he was credited with helping to substantially reduce the militant threat in the frontier city. His antiterrorism efforts were considered unusually energetic, and he had earned the ire of the Taliban, which asserted responsibility for his death.

The northwest's problems seem to be spreading. In the southern port city of Karachi this week, at least 72 people have been killed in a cycle of revenge attacks unleashed by the assassination of a local politician.

Karachi, which with a population of 16 million is the nation's largest city, has long been a cauldron for ethnic and sectarian tension, but those problems have been exacerbated by what city officials say is an influx of militants fleeing army offensives in the northwest.

Interior Minister Rehman Malik said the assassination was carried out by a nexus of the Pakistani Taliban and anti-Shiite groups, which appear to be working together in Karachi to foment unrest.

President Asif Ali Zardari has been absent for much of the week's turmoil. He left on Sunday for a previously scheduled trip to Europe that included a visit to his family's French chateau. His decision to leave the country while millions suffered amid the floods has generated widespread criticism, as has his choice to go ahead with a meeting with British Prime Minister David Cameron.

Staff writer Karen DeYoung in Washington and special correspondent Haq Nawaz Khan in Peshawar contributed to this report.

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