The White House described Baitullah Mehsud as "a murderous thug"
There are growing indications that Pakistan's most wanted man, Taliban leader Baitullah Mehsud, has been killed by a US missile.
A Mehsud aide reportedly confirmed that he had died when a drone attacked the house where he was staying.
Pakistan's foreign minister said he was "pretty certain" he was dead, adding that he would be hard to replace.
Taliban leaders have gathered in South Waziristan to choose a successor, local sources have told the BBC.
Three names are under consideration, says Abdul Hai Kakar, a BBC reporter based in Peshawar.
Hakimullah Mehsud, Maulana Azmatullah and Wali-ur-Rehman were all mentioned as possible successors.
M Ilyas Khan, BBC News, Islamabad
Speculation about whether Baitullah Mehsud is dead or alive is rife across Pakistan - from the mountainous tribal territory of South Waziristan to the capital Islamabad.
But the ambiguity surrounding his reported death may well persist. Nobody has as yet been willing or able to confirm his demise.
The Taliban have a strategy of blocking traffic to any area where missiles hit, so that the number of casualties and the identities of the dead remain unknown. They often bury the dead immediately to remove evidence.
Whether he is dead or alive, there are three possible ways of getting some clarity.
Communication intercepts may well pick up some news from key sources; ground intelligence might yield clues, although the government denies it has sources on the ground; or the Taliban may announce his death and could even announce his successor.
People living close to the scene of the missile attack in South Waziristan told the BBC Baitullah Mehsud had been killed with his wife early on Wednesday.
The remoteness of the location is contributing to the delay in establishing the facts, the BBC's Orla Guerin reports from Islamabad.
But Pakistani Foreign Minister Shah Mahmood Quresh told the BBC: "I think it's pretty certain now he's dead.
"Various government agencies have reported so, his own followers have said so, there are people who've been to the funeral and are witness to the burial."
He said Baitullah Mehsud's apparent death was a "significant development".
"He had quite a firm grip on various factions ... and there is no other personality who could replace him."
A spokesman for US President Barack Obama said the US could not confirm the death, but added that "there seems to be a growing consensus among credible observers that he is indeed dead".
The White House spokesman, Robert Gibbs, described Baitullah Mehsud as "a murderous thug", and said Pakistani people would be safer if he was dead.
Previous reports of Baitullah Mehsud's death have proved to be unfounded. He was said to be suffering from an illness for which he was taking medication.
South Waziristan is a stronghold of the Taliban chief, who has been blamed by Pakistan for a series of suicide bomb attacks in the country.
'Hit on the roof'
Kafayat Ullah, described as an aide to Baitullah Mehsud, told the Associated Press by telephone on Friday that his leader had been killed along with his second wife by a US missile. He gave no further details.
The missile fired by the US drone hit the home of the Taliban chief's father-in-law, Malik Ikramuddin, in the Zangarha area, 15km (9 miles) north-east of Ladha, at around 0100 on Wednesday (1900 GMT Tuesday).
At the time of the attack, the Taliban leader was said to be on the roof, local people told our Peshawar reporter.
Some who had reportedly seen his body said that it had been half-destroyed by the blast.
Baitullah Mehsud was buried in the nearby village of Nardusai, the witnesses told our reporter.
Several of Baitullah Mehsud's relatives were also injured, local people told the BBC earlier.
One factor complicating verification of his death is the lack of photographs of the Taliban leader.
When the BBC's Syed Shoaib Hasan went to interview him in South Waziristan in May 2008, he found himself sitting down before a short, plump, bearded man, reluctant to allow his picture to be taken.
Pakistan's interior minister, Rehman Malik, told the BBC that even if DNA could be recovered at the scene, the authorities did not have a sample from a male relative of the Taliban leader to compare it with.