The greatest casualty of the U.S. operation to kill Osama bin Laden, besides the al Qaeda leader himself, may be the U.S.-Pakistani relationship.

That the mastermind of the September 11, 2001, attacks was found in a three-story house in a Pakistani town prompted umbrage in the U.S. capital, where lawmakers said U.S. aid may hinge on what, if anything, Pakistan knew or suspected of bin Laden's whereabouts.

The bin Laden bombshell came as the two countries seek to repair ties frayed by friction over U.S. drone attacks on militants on Pakistan's border with Afghanistan and Pakistan's six-week imprisonment of a CIA contractor this winter.

While President Barack Obama and aides said Pakistan had helped lead them to bin Laden's doorstep in a complex in the city of Abbottabad they also said it was fair to question U.S. aid.

In the U.S. Congress, the questions were sharp.

"The United States provides billions of dollars in aid to Pakistan," said Senator Frank Lautenberg, a fellow Democrat on the Senate Appropriations Committee that apportions government spending. "Before we send another dime, we need to know whether Pakistan truly stands with us in the fight against terrorism."

Since 2001, Congress, which holds the U.S. government purse strings, has approved about $20 billion for Pakistan in direct aid and military payments, making it one of the top recipients of U.S. aid according to the Congressional Research Service.

Much of it been spent on building a close relationship with Pakistan's military with the intention that it would help the United States fight militants like bin Laden and stabilize a nuclear-armed Pakistan facing its own militant insurgency.

"This is going to be a time of real pressure" on Pakistan "to basically prove to us that they didn't know that bin Laden was there," said Senate Homeland Security Committee Chairman Joseph Lieberman, a Democrat turned independent who often sides with opposition Republicans on national security matters.

"In terms of the military aid ... support for that will depend on how Pakistan answers some of these questions, which need to be asked, about the presence of bin Laden in such a central location," said Senate Armed Services Chairman Carl Levin, a Democrat.

SERIOUS QUESTIONS

While Abbottabad is only about 40 miles (65 km) on a map from the Pakistani capital of Islamabad, the drive takes anywhere from two to five hours, depending on traffic, and the final stretch is through mountainous terrain.

The town is the headquarters for the Pakistan army's Baluch Regiment and one former resident said the life of the town largely revolves around the military, including the Kakul academy, which is Pakistan's equivalent to West Point.

The United States and Pakistan have had an uneasy alliance since the September 11, 2001 attacks, with Washington pushing Islamabad to take stronger action against suspected militants launching attacks on U.S. troops in neighboring Afghanistan.

Even Democratic Representative Howard Berman, an author of a 2009 law that expanded civilian aid to Pakistan by $1.5 billion a year over five years, said he had doubts about the military aid even before bin Laden was killed.

"I am more alarmed about the fact that we are funding and helping to equip a military that doesn't seem to have a view of its enemies that is the same as our view," he told Reuters.

Berman described a general pattern emerging with Pakistan maintaining ties with militants such as the Haqqani network and "refusing to take on the Afghan Taliban" as well as criticising the use of U.S. drone planes to attack militants.

"All this raises serious questions about what we are doing, with what is well close to 2 billion (dollars) a year in military assistance to the Pakistan military," Berman said.

FADING TO BLACK?

Asked if it was plausible Pakistani authorities had no idea bin Laden was in Abbottabad, former U.S. Secretary of State Condoleezza Rice said top leaders might have been ignorant but left open the possibility lower-level people may have known.

"I am surprised that they found him where they did, so close to Islamabad, but it is to me conceivable that at least top-ranking people didn't know that he was there," she told Reuters.

"I think a lot is going to come out over the next several weeks about what Pakistani cooperation looked like and that's an extremely important part of this," Rice said. "We need to understand that better."

Analysts stressed that ties already were strained.

"At a time when United States-Pakistan relations are going south in a hurry over aid, Afghanistan, and U.S. intelligence operations inside Pakistan, bin Laden's death leaves more questions on the table than answers," Shuja Nawaz, an expert on the Pakistan military, wrote on www.foreignpolicy.com.

Nawaz said if the operation was not coordinated with the Pakistanis -- and U.S. officials said they told no foreign governments in advance -- the relationship could suffer.

"If this operation was carried out in close cooperation with the United States, then the trajectory of this declining relationship may be reversed." he said. "If not, then the velocity of the decline will increase at a time when the mood in Washington seems to be shifting to black toward Pakistan, on the Hill and also in parts of the Obama administration.