Kabul - Defense Secretary Leon E. Panetta declared Saturday that the United States was "within reach" of defeating al-Qaida as a terrorist threat, but that doing so would require killing or capturing what he called the group's 10 to 20 remaining leaders.

Heading to Afghanistan for the first time since taking office this month, Panetta said that intelligence uncovered in the American raid in Pakistan that killed Osama bin Laden in May showed that 10 years of U.S. operations against al-Qaida had left it with fewer than two dozen key operatives, most of whom are in Pakistan, Yemen, Somalia and North Africa.

"If we can be successful at going after them, I think we can really undermine their ability to do any kind of planning to be able to conduct any kinds of attack on this country," Panetta told reporters on his way to Afghanistan aboard a U.S. Air Force jet, and that was why he believed the defeat of al-Qaida to be "within reach."

Panetta's comments were the most detailed recent assessment of al-Qaida's strength by a senior U.S. official. They follow President Barack Obama's decision to withdraw 30,000 troops from Afghanistan over the next year and a half, a move that he said was possible in part because of the damage inflicted on al-Qaida and its allies in Afghanistan and Pakistan.

Some terrorism experts were skeptical that al-Qaida has been so weakened that it no longer poses a threat to the United States or its allies.

"It is certainly true that al-Qaida's leadership has been significantly eroded over the past two years," said Bruce Hoffman, a terrorism expert at Georgetown University, but "there is no empirical evidence that either the appeal of its message or the flow of recruits into its ranks has actually diminished."

Nor is weakening al-Qaida likely to have much effect on the Afghan insurgency.

The Afghan conflict is driven by homegrown insurgent groups such as the Taliban that do not necessarily rely on assistance from al-Qaida to carry on their fight against U.S. and NATO troops. A relatively small number of fighters directly linked to al-Qaida are thought to be in Afghanistan, although Taliban offshoots such as the Pakistan-based Haqqani network are thought to have closer links with the organization.

The Taliban movement has a primarily domestic agenda that differs from the global holy war espoused by al-Qaida, and links between the two groups have loosened considerably in the nearly 10 years since the Taliban gave sanctuary to bin Laden after the Sept. 11 attacks.

Panetta, a former California congressman who headed the CIA before being chosen by Obama to replace Robert M. Gates at the Pentagon, did not estimate how long it might take to defeat al-Qaida, and he acknowledged that it would take "more work."

Panetta said during his confirmation hearings last month that al-Qaida had been severely damaged, but he had not said that it was nearing defeat.

The CIA and the military's Joint Special Operations Command have kept lists of senior terrorist leaders for years, adding new names as individuals on the list were killed or captured. It was unclear whether Panetta was indicating that the U.S. now believes it is nearing the end of the known militant leaders.

"Now is the moment following the death of bin Laden to put maximum pressure on them, because I do believe that if we continue this effort we can really cripple al-Qaida as a threat to this country," he said.

The U.S. believes that Ayman Zawahiri, the Egyptian who succeeded Bin Laden as al-Qaida's top leader, probably is hiding in Pakistan's Federally Administered Tribal Areas, the remote and largely ungoverned region along the border with Afghanistan where various militant groups now operate, Panetta said.

But getting help in finding him from Pakistan, which has severely scaled back cooperation with the U.S. on drone strikes and other operations since the bin Laden raid, could be more difficult than ever.

Before bin Laden was killed by U.S. troops in the garrison town of Abbottabad, Pakistani officials had for years dismissed U.S. claims that the Saudi militant was hiding in their country. Since the raid, which was undertaken without notice to Islamabad, Pakistan has halted or reduced most joint operations with the U.S.

In one of his last meetings as CIA director, Panetta said, he told the head of Pakistan's intelligence service that the U.S. had a list of targets that it wanted help in pursuing.

Zawahiri "is one of those we would like to see the Pakistanis target along with our help," he said.

At least one senior al-Qaida operative, Ilyas Kashmiri, was killed recently in a U.S. drone strike in Pakistan, a U.S. official said last week.

Panetta said that Yemen, not Pakistan, poses the most potent threat of terrorist attacks on America, from an al-Qaida offshoot known as al-Qaida in the Arabian Peninsula.

The group has gained strength in recent months as unrest has swept through Sana, the Yemeni capital, and large swaths of its rugged hinterlands.

The administration last week revealed that it had captured and interrogated Ahmed Abdulkadir Warsame, an alleged Somali militant with ties to the group, on a boat traveling between Yemen and Somalia. The U.S. has also targeted Anwar al-Awlaki, a U.S. citizen hiding in Yemen, with a drone strike but missed killing him.

"There's no question that when you look at what constitutes the biggest threat in terms of attacks on the United States, more of that comes from Yemen and from people like Awlaki," Panetta said.

Gen. David H. Petraeus, who is stepping down as the top commander in Afghanistan this month to take over Panetta's old job at the CIA, echoed Panetta's comments while speaking to reporters later in Kabul, the Afghan capital.

The al-Qaida offshoot in Yemen "is the one to keep an eye on now," he said.

"There has been enormous damage done to al-Qaida in the Federally Administered Tribal Areas ... and it does hold the prospect of really a strategic defeat" of al-Qaida, he said, noting that the organization's offshoots have suffered substantial setbacks in Iraq.