AFP - The United States hailed a joint operation with Pakistan that captured the Taliban's top commander as a "big success" as military forces continued a major offensive in Afghanistan's south.

On Wednesday, the Pakistani military confirmed the capture of Mullah Abdul Ghani Baradar, second only to Mullah Mohammed Omar in the Afghan Taliban hierarchy, without detailing the circumstances of his detention.

The confirmation unleashed a burst of congratulatory statements by exultant US officials, two days after the first reports that Baradar was seized in Karachi last week in a secret operation involving both Pakistani and US intelligence officials.

"It is a big success for our mutual efforts in the region," White House spokesman Robert Gibbs said.

Baradar ran the Taliban's war against the United States in Afghanistan and was privy to what experts said was a "gold mine" of information.

He is the most important Taliban figure to be captured since a 2001 US-led invasion ousted the Islamist movement from power in Afghanistan for sheltering Osama bin Laden and other Al-Qaeda leaders blamed for the September 11 attacks.

It was not known what, if anything, he has disclosed to his interrogators but analysts said his responsibilities spanned the gamut from strategy to finances to contacts and ties with other militant groups.

His capture in the sprawling Pakistani port city of Karachi signaled a dramatic change in direction for Islamabad, which has long resisted US pressure to take action against Taliban leaders operating from its territory.

Profile of Baradar: Taliban founding father

News of his capture came as Afghan troops raised their flag over a badly damaged market in the Taliban stronghold of Marjah in southern Afghanistan, where a major military offensive is being waged.

The ceremony provided a symbolic boost in one of the biggest pushes against the Taliban since the 2001 US-led invasion of Afghanistan.

Six foreign soldiers have been killed so far in Operation Mushtarak ("together" in Dari), a huge offensive testing a US-led counter-insurgency strategy that aims to follow up battlefield success with civil security and reform.

Afghan and Western forces reportedly came under fire from Taliban fighters hiding behind human shields, though militants denied hiding behind civilians.

"We have never used civilians as human shields, we do not use our own people as human shields," Yousuf Ahmadi told AFP by telephone from an undisclosed location.

Wednesday, President Barack Obama also met with senior cabinet and military officials to discuss the ongoing offensive and the capture of Baradar.

Joining Obama were the US Secretary of State Hillary Clinton and Defense Secretary Robert Gates, as well as Vice President Joe Biden.

General Stanley McChrystal, ground commander of the 113,000 US-led and NATO troops fighting against the Taliban across Afghanistan, was to join the meeting via video conference, the White House said.

Baradar's arrest was announced in Islamabad in a curt statement from Major General Athar Abbas, a Pakistani military spokesman.

"At the conclusion of detailed identification procedure, it has been confirmed that one of the persons arrested happens to be Mullah Baradar," he said.

In Kabul, Richard Holbrooke, the special US envoy to Afghanistan and Pakistan, said Baradar "got caught" and called it "a significant development."

But Holbrooke, like others, refused to go into detail about the operation.

"I am not going to join any speculation," he said. "I am not in a position to share. This is an intelligence matter, but we commend the Pakistanis for their role in this and it is part of the deepening co-operation between us."

US officials had long complained that Islamabad's spy service was failing to crack down on the Afghan Taliban and Baradar's detention exposed the presence of Afghan Taliban leaders in Pakistan.

Despite Pakistani denials, the leadership is widely reported to be headquartered in Quetta, the capital of southwestern Baluchistan province.

"This arrest is very significant. It is a major setback for the Taliban. It will have a demoralizing effect on them, something in the favor of both the United States and Pakistan," said Pakistani political analyst Hasan Askari.

"It will also help advance President Obama's new strategy in Afghanistan. There appears to be a shift in the Pakistan army's policy."