The seizure of the Afghan Taliban's top military leader in Pakistan represents a turning point in the U.S.-led war against the militants, U.S. officials and analysts said.

The arrest of Mullah Abdul Ghani Baradar represents the most significant Taliban capture since the terrorist attacks of September 11, 2001, a senior Obama administration official said Tuesday.

Baradar has been a close associate of Osama bin Laden's and is seen as the No. 2 figure in the Afghan Taliban, behind Mullah Muhammar Omar.

"If anyone would know where the senior leaders are of al Qaeda and the Taliban, then Baradar is someone who would be privy to that kind of information," said M.J. Gohel, executive director of the Asia-Pacific Foundation.

It's "major success for the CIA" and "a major blow for the Taliban," Gohel said.

The United States has tried to target Baradar for years, a senior U.S. official said.

The arrest also represents a "new level of cooperation" between Pakistani and American forces working to rout the Taliban, said U.S. Sen. John Kerry, chairman of the Senate Foreign Relations Committee and co-author of legislation designed to improve cooperation between Pakistan and the United States.

Described as a savvy and modern military leader, Baradar was arrested in the southern Pakistani city of Karachi several days ago, a senior Pakistani intelligence official said. The official asked not to be named because he is not authorized to speak to the media.

Baradar is being held in joint custody and investigated by both the CIA and Pakistan's Inter-Services Intelligence agency, another senior Pakistani source said.

Qari Yousuf Ahmadi, a Taliban spokesman in Afghanistan, denied that Baradar had been captured. He said Baradar is continuing his operations and is in Afghanistan.

Another Afghan Taliban spokesman, Zabiullah Mujahid, also denied Baradar had been arrested. He said reports of his arrest are designed to demoralize the Afghan Taliban.

Despite confirmation of the arrest by Pakistani sources, Pakistani Interior Minister Rehman Malik said he could not verify reports of the capture. He also denied reports the CIA and ISI conducted a secret raid that captured Baradar, saying the agencies share intelligence but that the CIA does not conduct raids on Pakistani soil.

Several raids in Karachi last week netted dozens of suspected Afghan militants, and intelligence agencies are in the process of verifying their identities, Malik said.

Baradar's arrest occurred as some 15,000 Afghan and NATO forces were battling the Taliban in the Marjah region of southern Afghanistan's Helmand province in the largest NATO offensive since the U.S.-led invasion of Afghanistan in 2001.

It also comes weeks after the CIA ratcheted up its operations against the Taliban in apparent response to a December suicide attack that killed seven CIA officers in eastern Afghanistan.

And the arrest comes amid reports of major successes for the United States in its battle against the Taliban and associated militants in Afghanistan and Pakistan.

The leader of the Pakistani Taliban, Hakimullah Mehsud, died recently after reports that a suspected U.S. drone strike targeted him in January, according to Taliban and Pakistani intelligence sources. The previous leader of the Pakistani Taliban, Baitullah Mehsud, also died in a suspected U.S. drone strike.

CNN national security analyst Peter Bergen called Baradar's capture a "huge deal," saying he is "arguably more important than Mullah Omar from a military point of view, because Mullah Omar really is more of a religious figure than an operational commander of the Taliban."

"This guy also is the No. 2 political figure in the Taliban. The fact that he was discovered in Karachi is very significant. Karachi is the largest city in Pakistan. It's a long way from where the war is being fought," Bergen said Monday on CNN's "Anderson Cooper 360

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