Pakistan's interior minister said on Sunday that he was "98 percent sure" senior al Qaeda operative Ilyas Kashmiri was killed in a US drone strike near the Afghan border.
US officials in Washington were sceptical over reports that Kashmiri, seen as one of the world's most dangerous militants, was dead.
A US National Security official said he could not confirm that he had been killed and another US official said it was doubtful.
"All ground intelligence shows that he is dead. What I can say is there is a 98 percent chance he is dead," Interior Minister Rehman Malik told Reuters.
"Since we do not have the body. We do not have DNA we need to confirm. This is the substantive evidence we are looking for."
That may not be possible since it is very difficult for Pakistani security forces to get to areas like South Waziristan where intelligence officials said Kashmiri was killed in a drone strike on Friday night.
After missile strikes by remotely-operated drone aircraft, militants often seal off the area then bury their comrades.
The elimination of Kashmiri would be another coup for the United States after American special forces killed Osama bin Laden in a garrison town close to Islamabad on May 2.
The killing of bin Laden aroused international suspicions that Pakistani authorities had been complicit in hiding him, and led to domestic criticism of them for failing to detect or stop the US team that killed him.
A senior Pakistani security official said: "It's almost confirmed that he is dead. Different sources confirmed it but we can't say it is 100 percent confirmed because we don't have the body."
He went on to say that Kashmiri was holding a meeting with other militants when the drone missile struck.
US doubts over claims of Kashmiri's demise may be further evidence of deep distrust between Pakistani and US intelligence services public pledges by Secretary of State Hillary Clinton and other American officials that relations had improved.
One intelligence official said that Pakistan had tipped off the Americans about the whereabouts of Kashmiri, whom the US Department of State has labelled a "specially designated global terrorist".
Kashmiri, said to be a former Pakistani military officer, has been linked to attacks including the 2008 rampage through the Indian city of Mumbai which killed 166 people.
A Pakistani television station quoted the group Kashmiri headed, Harkat-ul Jihad Islami (HUJI) which is allied to al Qaeda, as saying he had been killed and that it will avenge his death.
The SITE online monitoring service said the HUJI statement was posted on a jihadist forum it tracks. The US National Security official expressed doubts about the statement.
Its authenticity could not be independently verified.
Kashmiri was reported to have been killed in a September 2009 strike by a US drone. He resurfaced and gave an interview to Asia Times online correspondent Saleem Shahzad.
Shahzad disappeared from Islamabad a week ago. His body was found in a canal two days later with what police said were torture marks. The media and human rights groups have speculated that Pakistan's military intelligence agency may have had hand in the killing, an allegation it strongly denied.
Human Rights Watch said Shahzad had voiced concern about his safety after getting threatening telephone calls from Pakistani intelligence agents and was under surveillance since 2010.
Before his death, Shahzad wrote an article stating that Kashmiri's followers carried out a militant siege of the PNS Mehran naval base in Karachi last month which drew sharp public criticism of the Pakistani military.