By Arun Kumar

Washington, May 19 (IANS) While the United States has no proof that Pakistan's top leaders knew Osama bin Laden was hiding near the capital city of Islamabad, "somebody had to know," US Defence Secretary Robert Gates has said.

"It's my supposition, and I think it's a supposition shared by a number in this government, that somebody had to know, but we have no proof and no evidence," Gates told reporters at the Pentagon Wednesday.

Because of a lack of evidence, "it's hard to hold them accountable for it," he said.

But Gates and the chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, cautioned against holding back aid until Pakistan moves against militants within its borders, saying Islamabad is already paying for its inaction.

The ability of the United States to enter Pakistan, kill Osama bin Laden and leave without detection was a humiliation to Pakistanis, Gates said.

"If I were in Pakistani shoes, I would say I've already paid a price. I've been humiliated. I've been shown that the Americans can come in here and do this with impunity," he said.

Responding to a question about what should be done if someone in Pakistan's government did know, Gates said: "I think we have to recognize that they see a cost in that and a price that has been paid."

Mullen said that "with the evaluation of the sensitive site material, it's just going to take us awhile to see if there's anything else."

Gates also said he understands US lawmakers' frustrations with Pakistan, though he urged them to be patient.

"We have to proceed with some caution," Gates said. "We do need to be cognizant of the concerns on the Hill, and frankly, I think the Pakistanis need to be as well. But that said, we do have interests in common."

Mullen said the incident was a "humbling experience" for the Pakistanis that has led to "internal soul-searching."

Gates said he is frustrated by the Pakistani reluctance to go after Al Qaeda and Taliban elements in northwest Pakistan.

The raid on bin Laden's hideout has opened an opportunity to address that issue again but the Pakistanis have warned the United States that another such covert raid would not be tolerated, he said.