U.S. leaders credited Pakistan on Monday for its help against al-Qaida even as they raised the possibility that Pakistani officials may have known where Osama bin Laden had been hiding.

The conflicting messages reflected the frustrations that pervade ties between Washington and Islamabad: The U.S. needs Pakistan's cooperation to fight Islamic extremism but can't fully trust its ally.

The U.S. launched the operation that killed the al-Qaida leader without informing Pakistan, a risky move.

President Barack Obama's top counterterrorism adviser, John Brennan, acknowledged they were concerned that Pakistan could scramble fighter jets, after the two U.S. Chinook helicopters carrying special forces zeroed in on their target in Abbottabad, a city where three army regiments with thousands of soldiers are based. He said Pakistan was not contacted about the raid until after it was complete and the U.S. forces had left Pakistani airspace.

Brennan did not mention what contingency had been made for the possibility of a clash with Pakistani forces, which could have had a serious impact on relations most recently strained by the fatal shooting of two Pakistanis by a CIA operative.

Pakistan says it does not let foreign troops operate on its soil, including U.S. forces based in neighboring Afghanistan. While U.S. drone strikes are commonplace against militant targets in border regions of Pakistan, and hot pursuit of targets across has been reported, such a raid by U.S. ground forces deep inside Pakistan territory appears unprecedented.

Pakistan welcomed the killing of the al-Qaida leader, but many questions remain over how the most wanted man in the world could have hidden so close to a major military facility without its knowledge.

While Brennan went out of his way to credit Pakistan's help in fighting terrorism, he said it was "inconceivable" that bin Laden did not have a support network inside Pakistan. He left open the possibility it could have been "official" in nature and said they were closely talking to Pakistan about it.

U.S. officials say the three-story compound was custom built for bin Laden in 2005. Abbottabad lies just 35 miles from the capital, Islamabad.

Sen. Carl Levin, D-Mich., chairman of the Armed Services Committee, spoke bluntly. Pakistan's military and intelligence "have a lot of explaining to do," he told reporters.

"The Pakistani army and intelligence have a lot of questions to answer, given the location, the length of time and the apparent fact that ... this facility was actually built for bin Laden," he said.

Secretary of State Hillary Rodham Clinton, however, struck a different tone, saying Pakistan "has contributed greatly to our efforts to dismantle al-Qaida." She went so far as to say that cooperation with Pakistan "helped lead us to bin Laden and the compound in which he was hiding."

Pakistan formally abandoned its support for the Taliban in Afghanistan after the Sept. 11, 2001, attacks. In return for billions in U.S. aid, it sent tens of thousands of troops into its lawless tribal areas along the border where bin Laden and al-Qaida followers fled after U.S. forces invaded Afghanistan and toppled the Taliban. It was the first Pakistani military deployment in that volatile region in its independent history.

Pakistan also helped nab a number of top al-Qaida figures, including 9/11 mastermind Khalid Sheikh Mohammmed and attacks' facilitator Ramzi Binalshibh.

That help for the U.S. angered religious conservatives, and al-Qaida and Pakistani Taliban have target the nation's military and government in bloody attacks. Hoping to win Islamabad's confidence, the Obama administration has been trying to deepen the relationship, providing more civilian aid intended to boost the ailing Pakistan economy and strengthen its weak democracy.

But the suspicions run deep. Pakistan's military is widely assumed to be playing a double game by retaining links with top Afghan Taliban leaders. Last month, Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, accused Pakistan's military-run spy service of maintaining links with the Haqqani network, a major Afghan Taliban faction.