USMC Lt. General John Allen (L) and US Army General David Petraeus (C) greet US Secretary of Defense Leon Panetta after Panetta's arrival in Kabul, Afghanistan, July 9, 2011. [Photo/Agencies]

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

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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 Qaeda's leadership.

"We're within reach of strategically defeating al Qaeda 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 Qaeda as a threat to (the United States)."

Panetta declined to offer all the names of al Qaeda 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 Qaeda's Yemen-based affiliate, and Ayman al-Zawahri, who replaced bin Laden as the head of al Qaeda.

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 Qaeda in the Islamic Maghreb) in North Africa. Those are, if we can go after them, I think we really can strategically defeat al Qaeda," 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.