KABUL - New US Defense Secretary Leon Panetta arrived in Afghanistan on Saturday, saying he believed the strategic defeat of al-Qaida was within reach if the United States could kill or capture up to 20 remaining leaders of the core group and its affiliates.

Panetta, on his first trip since taking over the Pentagon on July 1, told reporters before arriving in Kabul that now was the time -- in the wake of the May killing of Osama bin Laden -- to intensify efforts to target al-Qaida's leadership.

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"We're within reach of strategically defeating al-Qaida and I'm hoping to be able to focus on that, working obviously with my prior agency as well," said Panetta, who ran the CIA until the end of June.

"Now is the moment following what happened with bin Laden, to put maximum pressure on them. Because I do believe that if we continue this effort that we can really cripple al-Qaida as a threat to [the United States]."

Panetta declined to offer all the names of al-Qaida leadership the United States was looking at. But he singled out two men: Anwar al-Awlaki, an American imam who has become a senior leader of al-Qaida's Yemen-based affiliate, and Ayman al-Zawahri, who replaced bin Laden as the head of al-Qaida.

Panetta said he believed Zawahri was living in Pakistan's tribal areas, and "he's one of those we would like to see the Pakistanis target."

"I would say somewhere around 10-20 key leaders that between Pakistan, Yemen, Somalia, AQIM (al-Qaida in the Islamic Maghreb) in North Africa. Those are, if we can go after them, I think we really can strategically defeat al-Qaida," he said.

Panetta added that the US military and the CIA were engaged in a number of operations focusing on militants in Yemen. He did not give specifics.

Improving US ties with Karzai?

Panetta's first trip to Afghanistan as defense secretary comes at a time of growing impatience in the United States with the nearly decade-old war, even as US President Barack Obama pushes ahead with plans for a faster-than-expected drawdown.

Critics say the US war strategy is undermined partly by Pakistan's failure to go after militants -- including those staging cross-border attacks in Afghanistan.

Panetta acknowledged Pakistani cooperation in going after some militants but added "we've got to continue to push [the Pakistanis] to do that."

"That's the key," Panetta said.

Critics also point to the strained relationship with Afghan President Hamid Karzai, who has been accused of failing to address rampant corruption.

Although Panetta has met Karzai several times before as CIA director, he expressed hope that Obama's decision to move top officials including himself into new roles linked to Afghanistan would help improve ties.

"Hopefully it can be the beginning of a much better relationship than what we've had [with Karzai] over the past few years," Panetta said.

Obama's withdrawal plans call for the first 10,000 of the nearly 100,000 US troops in Afghanistan to leave the country by the end of this year. Another 23,000 will pull out by the end of next summer, with the goal of gradually handing over lead security responsibility to Afghan forces by the end of 2014.

Panetta said the biggest challenge remained training Afghan forces in the months and years ahead.

"We've made good progress on that, but I think there's a lot more work to do of being able to transition responsibility to them," he said.

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