Security forces in Pakistan have arrested a former Taleban defence minister, intelligence officials say.
Mullah Obaidullah Akhund, said to be a leader in the Taleban insurgency in Afghanistan, was reportedly seized in the south-western city of Quetta.
The raid coincided with a visit by the US vice-president on Monday, officials say. However, Pakistani officials have not formally confirmed the arrest.
A Taleban spokesman denied Mullah Obaidullah had been captured.
Although Mullah Obaidullah's arrest has not been confirmed by the Pakistani authorities, numerous reports quoting unnamed intelligence officials and Taleban sources say he was detained on Monday.
One Pakistani intelligence official said the raid had been a planned operation and had not been linked to US Vice-President Dick Cheney's visit to the region.
Mullah Obaidullah would be the most senior Taleban leader to have been captured since Afghanistan's former hard-line rulers were forced out of power in 2001.
A number of other suspected Taleban figures were also arrested in Quetta, the capital of Balochistan province, this week, officials said.
The arrest has also been confirmed by US intelligence officials quoted in the New York Times newspaper.
But Pakistani Interior Minister Aftab Sherpao said he was "not aware" of the arrest.
A Taleban spokesman, Qari Mohammad Yousuf, told the BBC that reports of Mullah Obaidullah's arrest were baseless.
He said the Taleban leadership was based in Afghanistan.
The denial could not be verified independently, and contradicted Taleban sources talking to the Reuters news agency by telephone from undisclosed locations.
They said Mr Akhund had indeed been caught at the home of a relative in the Balochistan capital.
The BBC's Charles Haviland in Kabul says there is intense interest the fate of Mullah Obaidullah in Afghanistan.
Mr Cheney asked Pakistan's President Musharraf to do more on terror
Our correspondent says he is well known because of his senior political and military leadership role in the Taleban.
Mullah Obaidullah was defence minister in the Taleban government before 2001 and the second of three top deputies of the Taleban's leader, Mullah Omar, who remains at large.
He is effectively the Taleban's number three, seen as the Taleban's military chief and the man to whom the other Taleban commanders answer, our correspondent says.
Mullah Obaidullah is also a religious scholar, educated in madrassas in two Pakistani cities.
Soon after the fall of the Taleban he actually surrendered to the Afghan government along with some colleagues. But he was soon released as part of an amnesty deal.
In the past, rumours of senior Taleban being arrested have proved wrong, our correspondent says, and now Afghans will be watching keenly to see if this time Mullah Obaidullah is indeed behind bars.
Mr Cheney's visit to the region on Monday came amid increasing concern about insurgent activity in several areas of Afghanistan.
In talks with Pakistani President Pervez Musharraf, he urged Pakistan to do more to combat the Taleban near the Afghan border, but also praised its role in the "war on terror".
US defence officials told Congress on Thursday that the threat from the Taleban is greater now than a year ago.
Some 27,000 US troops have been deployed in Afghanistan, the highest number since the invasion of 2001, amid fears of a spring offensive by the Taleban and its allies as the snows clear.
On an early version of this report, we inadvertently published a photograph of Qari Obaidurahman Qarizada, wrongly identifying him as a former Taleban defence minister. We removed the picture as soon as we became aware of the error. We apologise for any misunderstanding it may have caused.