The notice said Pakistani authorities recently notified the family of Mullah Obaidullah Akhund that he died in a Karachi prison of a heart-related illness, a report by the BBC said.
Obaidullah -- age unknown -- was defense minister in Afghan's Taliban government from the mid 1990s until the Taliban were overthrown by U.S. forces and their allies in 2001. He was ranked as one of three senior deputy leaders to Mullah Omar, the spiritual leader of the Taliban movement.
He later became a military commander within Taliban forces.
The Web site statement by Taliban spokesman Zabiullah Mujahid said Obaidullah relatives in Afghanistan's Kandahar province aren't sure that the Pakistani story is true or that there was some kind of torture that led to his death.
Taliban leaders are asking for more information from Pakistani authorities, the statement said.
The BBC said the Taliban leadership had at one time wanted Obaidullah released from prison before it entered into peace talks with the Afghan government.
He was arrested first in Quetta in 2006 but released nine months later, in 2007, a report by the Pakistani Daily News newspaper said in early 2008.
After his release he fled to Afghanistan but returned to Pakistan in the first week of January 2008. He contacted several influential people with links to banned militant organizations and convinced them to donate money to the Taliban, the Daily News report said.
Obaidullah was rearrested in early February 2008 and over the next several years Pakistani police made few public statements about where he was being held.
The importance of his final arrest was underlined in a report by The Long War Journal, based in Washington, in March 2007. The report said "he was grabbed after flurry of diplomatic pressure" between Pakistan and the U.S. government.
The Long War Journal report said Obaidullah "can provide significant intelligence" to NATO, Afghanistan and Pakistan. "A man of his stature may know the whereabouts of Mullah Omar, Mullah Dadullah, Osama bin Laden, Ayman al-Zawahiri and other senior Taliban and al Qaida leaders."
His final arrest came during a period in which the United States was questioning Pakistan's determination to fight the War on Terror.
The arrest of Obaidullah occurred as the United States and Britain were pressuring Pakistan to take meaningful action against the Taliban and al Qaida camps, leaders and operatives inside its borders. But the issue is whether Pakistan would give access to NATO and Afghan intelligence and act on intelligence once they had it, the Long War Journal said.
"Pakistan has a history of making arrests and conducting strikes on Taliban and al-Qaida camps when the political situation suits it," the report said.
The announcement of Obaidullah's death comes as 19 Taliban fighters in Herat province, around 400 miles from Kabul, surrendered to the Afghan government, China's Xinhua news agency said.
"Today, 19 armed Taliban rebels, including their commander namely Syed Zia, laid down their arms in Chishti Sharif district and surrendered to the government," the Xinhua report said.
"We appreciate their decision and hope other oppositions to follow the step," Gov. Daud Sabah told Xinhua.
He said the former commander of insurgents Syed Zia was serving as Taliban rebel governor in Chishti Sharif district before joining the peace process.
Taliban militants fighting the government have yet to make a comment, Xinhua said.