The capture of a top Taliban commander may have dealt the hard-line Islamist insurgents a serious blow, analysts say, especially when coupled with a major coalition offensive in southern Afghanistan.

Mullah Abdul Ghani Baradar, the No. 2 man in the Afghan Taliban's hierarchy, was arrested last week in the Pakistani port city of Karachi after a joint U.S.-Pakistani raid that was kept secret until now.

Hanif Atmar, the Afghan Interior Minister, said Baradar's arrest will severely impair the Taliban's ability to mount attacks on coalition and Afghan government forces.

"Given the significance of this individual there will be a significant impact on the ability of the insurgency," he said.

He is the most important Taliban leader to be captured since the war in Afghanistan began in 2001 and his arrest could mark a significant shift in the insurgents' fortunes.

The Taliban have used Pakistan's frontier district as a hideout for years, crossing the long, porous Afghan border with apparent impunity to mount attacks in southern and eastern Afghanistan.

But Baradar's arrest could be a signal that Pakistani security forces are now working to deny Afghan insurgents a safe haven.

That could encourage other insurgent leaders sheltering in Pakistan to negotiate with the Afghan government -- a development increasingly seen as key to ending the eight-year war.

Baradar, a founding member of the Taliban, was the second in command behind supreme leader Mullah Mohammad Omar and was said to be in charge of the movement's leadership council, widely thought to be based in Pakistan. He acted as the link between Mullah Omar and his field commanders.

Baradar has been in detention for more than 10 days and is talking to interrogators, Pakistani intelligence officials told The Associated Press Tuesday, speaking on condition of anonymity.

One said several other suspects were also captured in the raid. He said Baradar had provided "useful information" to them which they had shared with their U.S. counterparts.

The White House would not confirm Baradar's capture. Spokesman Robert Gibbs told reporters the fight against extremists involves sensitive intelligence matters and he believes it's best to collect that information without talking about it.

News of his capture emerged on the fourth day of Operation Moshtarak, a massive offensive in Helmand province involving some 15,000 U.S., British, Afghan and Canadian troops.

U.S. Marines and Afghan National Army soldiers were moving methodically through the town of Marjah, 610 kilometres southwest of Kabul, Wednesday, conducting house-to-house searches, removing bombs and booby-traps as they moved through town.

Marine spokesmen say Taliban resistance in the town seemed to be growing more disorganized and poorly co-ordinated.

Nevertheless, Taliban have not given up. Insurgent snipers hiding in haystacks in poppy fields exchanged fire with Marines and Afghan troops as they swept south.

Insurgents tried but failed to shoot down an Osprey aircraft with rocket-propelled grenades as Cobra attack helicopters fired missiles at Taliban positions, including a machine-gun bunker.

A Taliban spokesman told The Associated Press however that insurgents retain control of the town and that coalition forces who "descended from helicopters in limited areas of Marjah" were now "under siege."

Marjah has an estimated 80,000 inhabitants and was the largest southern town under Taliban control. NATO hopes to rush in aid and public services as soon as the town is secured to try to win the loyalty of the population.

Baradar's capture is expected to hurt the Taliban, but former members of the movement and security experts said his arrest was far from a decisive blow. They said he will likely be quickly replaced and that local commanders have significant autonomy from leaders based in Pakistan.

Nevertheless, the capture is likely to cause short-term disruption, since Baradar was the day-to-day commander of the Taliban and his successor would not have the same prestige.

"It's a great tactical success that the coalition forces should be pleased with, but by no means is it the beginning of the end," Will Hartley, an analyst at Jane's Terrorism and Insurgency Center in London, told AP. "This will have a noted effect on the short-term ability of the Taliban to operate the way it was. However, it has proved itself a resilient organization."

Pakistan's spy agencies have long been accused of protecting top Afghan Taliban leaders -- many of whom are believed to have fled to Pakistan during the U.S.-led invasion -- to use them as tools to counter Indian influence in Afghanistan.

With mounting U.S. casualties in across the border, American officials have pressured Pakistan to target the group's leaders. Security forces here have largely resisted doing so, even while attacking Pakistani Taliban groups blamed for scores of terror attacks.

U.S. and Pakistani officials did not say what led them to Baradar or give details of the raid, triggering speculation that he may have been handed over by Pakistani intelligence officials as part of a trade off in negotiations over the future of Afghanistan or betrayed by other members of the Taliban.

"If Pakistani officials had wanted to arrest him, they could have done it at any time," said Sher Mohammad Akhud Zada, the former governor of Afghanistan's Helmand province and a member of the Afghan parliament. "Why did they arrest him now?"

The arrest could mark a shift in strategy by Pakistan's powerful Inter Services Intelligence agency from protecting or turning a blind eye to the Afghan Taliban to arresting them.

"The Pakistani government have realized that the Taliban is too much of a threat to them, they've decided they've got to draw some red lines for both Pakistani and Afghanistan Taliban," said Michael Clarke, director of the Royal United Services Institute, a military think-tank in London. "They decided they need to be seen to take the Taliban on, they need to push them back."

With files from The Associated Press