US and Pakistani spies have captured the Taliban's top military commander, US media reported, dealing a blow to the Afghan insurgency just as American-led troops pressed an offensive against a southern stronghold.
Mullah Abdul Ghani Baradar was arrested in Pakistan's sprawling port city of Karachi "several days ago" by US and Pakistani intelligence services and was being interrogated, The New York Times and other media said, citing unnamed officials in both countries.
His arrest, if confirmed, signalled a major success for Washington's bid to persuade Pakistan to move aggressively against Islamist militants operating on both sides of the Afghan-Pakistani border.
US officials previously complained that Islamabad's spy service was failing to crack down on Afghan Taliban figures based in Pakistan.
Baradar's reported capture emerged as 15,000 US, Nato and Afghan troops staged a major assault to capture the Taliban bastion of Marjah in southern Afghanistan, key to Washington's new strategy for turning around the troubled war.
But the Taliban denied Baradar's capture and accused US officials of trying to deflect attention from resistance they are facing in Marjah.
"We strongly reject the reports of his arrest," Taliban spokesman Yousuf Ahmadi said by telephone from an undisclosed location.
"He is currently in Afghanistan, where he is leading all jihadi activities... The sole goal of such baseless reporting and propaganda is to make up for the failure in Marjah."
The Pentagon declined to openly discuss the commander's reported arrest.
"I can neither confirm nor deny Baradar's capture", Pentagon spokesman Colonel Dave Lapan told reporters.
The US previously has taken out other Taliban figures, often in missile strikes by unmanned drones, but the insurgency has bounced back in each case and the possible effect of Baradar's detention remained unclear.
Col Lapan said going after Taliban leaders in the past has tended to disrupt the group's operations at least in the short term.
"It has an impact, an immediate impact to their operations when we kill or capture their senior leaders but we have seen too that they then push successors into their place," he said.
The Pentagon spokesman also acknowledged US relations with Pakistan have been "improving" after years of friction.
Islamabad's ties with the US are controversial in Pakistan, where Interior Minister Rehman Malik stopped short of confirming or denying that Baradar was in custody, but criticised the report as "propaganda".
The Afghan-born Baradar is known as a powerful military chief and trusted aide to the Taliban's one-eyed and elusive leader Mullah Mohammad Omar.
"Mullah Baradar was a close friend of Mullah Omar and both are of the same age group. He was among some 30 people considered founders of the Taliban movement," said Pakistan-based Taliban expert Rahimullah Yusufzai.
Born in the southern Afghan province of Uruzgan to the Popalzai tribe, Baradar fought in the war - covertly backed by the US and Pakistan - to expel the Soviet forces that occupied Afghanistan in the 1980s.
When the Taliban rose to power in 1996, Baradar's ties to Omar helped secure him the position of deputy defence minister, before the hardline Taliban regime was toppled by the US-led invasion in 2001 for sheltering al-Qaeda.
Officials from Pakistan's military, which controls the country's premier spy agency, were not available for comment and senior police in Karachi said they had no information.
In Washington, the Central Intelligence Agency and the White House declined to discuss the reports.
Baradar is in charge of the Taliban's military operations and leadership council, and was a reportedly close associate of al-Qaeda leader Osama bin Laden before the September 11, 2001 attacks on the US.
"If he were taken off the battlefield, it would deal a major setback to the Afghan Taliban and be a personal blow to Mullah Omar, who has relied heavily on him for years," a US counter-terrorism official told ABC television.
A Pakistani expert on the Taliban, Rahimullah Yusufzai, expressed some doubt about the US reports, saying fighters can operate under different names.
"If confirmed, his arrest would be a psychological, political and symbolic setback for the Taliban. But it will not end the war nor will Taliban lay down their arms," he said.