The Taliban's top military commander was captured in Pakistan, U.S. and Pakistani officials said on Tuesday, but the move may not deal a decisive blow to a group putting up fierce resistance to a NATO offensive.

The Afghan Taliban denied that Mullah Abdul Ghani Baradar was captured in a joint raid by Pakistani and U.S. spy agencies.

Washington hopes Mullah Baradar's capture will at least temporarily weaken the Taliban-led insurgency in Afghanistan, where U.S. Marines are leading one of NATO's biggest offensives in the southern militant stronghold of Marjah.

I would call it significant, a U.S. official told Reuters on condition of anonymity. But even when you get their leaders, they've shown an amazing resilience to bounce back. It's an adaptive organization.

A second U.S. official confirmed the capture.

A Pakistani security official who declined to be identified told Reuters: Yes, it's true. He has been arrested. He's in our custody.

He declined to give details and Pakistani military and government spokesman were not available for comment. The New York Times said the capture took place in the southern Pakistani port city of Karachi.

A large number of people have fled the NATO operation in Afghanistan and crossed into Pakistan. We have picked up many. We are investigating them and right now we cannot divulge their identity, Pakistani Interior Minister Rehman Malik, asked by reporters about the arrest, said.

Pakistan's role may signal a new level of cooperation against Taliban leaders behind the Afghan insurgency. Pakistan has long resisted U.S. calls for a crackdown.

We continue to look for opportunities to coordinate across the border, the second American official said. We appreciate the help we get.

IMPACT UNCLEAR

Kamran Bokhari, regional director of global intelligence firm STRATFOR, said Mullah Baradar was a key deputy of Taliban leader Mullah Omar and a senior figure in the group's leadership council.

It's not clear though that arrest will have a major impact on the battlefield, he said. I suspect the Taliban gave up this guy to the Pakistanis who then gave him to the Americans in exchange for some concessions on Afghanistan and India.

A Taliban spokesman said Mullah Baradar was still in Afghanistan actively organizing the group's military and political activities.

He has not been captured. They want to spread this rumor just to divert the attention of people from their defeats in Marjah and confuse the public, Zabihullah Mujahid told Reuters, referring to the NATO offensive.

In a sign of how tough it is to gather intelligence on the Taliban, there have been conflicting reports on who is winning the battle in Marjah, a breeding ground for militancy and poppy cultivation which Western states say finances the insurgency.

Some Marine units have been bogged down by heavy Taliban gunfire, sniper attacks and booby traps, though a Marine spokesman said they were making steady progress.

BIG UPHEAVAL?

Ghulam Mahaiuddin Ghori, a senior Afghan National Army general for the southern region, said many Taliban militants had escaped and Afghan forces had reached the stage where they are searching houses for arms and ammunition.

We have asked the shopkeepers to reopen their business and have urged those villagers who had left (prior to the launch of the operation) to return to their houses, he told Reuters.

Although the capture of Mullah Baradar may be a big morale boost to Washington after a suicide bomber killed seven CIA employees at a U.S. base in Afghanistan in December, getting him to give up valuable intelligence is another matter.

Especially when it comes to clues on the whereabouts of reclusive Taliban chief Mullah Mohammad Omar, who gave him the nickname Mullah Baradar (brother), in a sign of trust.

In something very rare in the austere Taliban leadership culture, Mullah Omar and Mullah Baradar were known to crack jokes in front of comrades.

Waheed Mozhdah, an Afghan analyst who served as an official during the Taliban's rule, said the capture of Baradar would create turmoil in the Taliban, a group which made a steady comeback since being toppled in a U.S.-led invasion in 2001.

If true, that will be an upheaval and a big development, he said.

(Reporting by Peter Cooney in Washington, Zeeshan Haider in Islamabad and Faisal Aziz in Karachi; Writing by Michael Georgy and Nick Macfie; Editing by Bryson Hull and Ron Popeski)

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