U.S. intelligence agencies believe Pak Tehrike-e-Taliban leader Hakimullah Mehsud, might have been killed in a firefight with a rival faction weeks ago.
Militants appointed Hakimullah to replace the group's previous leader, Baitullah Mehsud, who was killed by a missile fired from a CIA-piloted drone aircraft in his South Waziristan stronghold on the Afghan border on Aug. 5.
Hakimullah's death, which officials said has yet to be confirmed definitively, would be another setback for Pak Tehrik-e-Taliban.
"We're pretty clear that we think he's dead," a U.S. defense official said of Hakimullah.
A counterterrorism official said: "While there's no final confirmation of his death, it's a distinct possibility."
The officials said Hakimullah was believed to have been shot weeks ago during a clash with a rival group in South Waziristan.
U.S. intelligence agencies are still reviewing information to make a final call on his death.
Baitullah Mehsud, who led an alliance of 13 militant groups known as the Taliban Movement of Pakistan, was blamed for a series of suicide bombings in Pakistan, including the one that killed former Prime Minister Benazir Bhutto in 2007.
Hakimullah had been described as even more aggressive than Baitullah.
US officials believe the Taliban has been weakened by infighting between factions vying to take command since Baitullah Mehsud's death.
"The point is there is a succession crisis going on," the defense official said, adding there were other contenders to lead the movement. "There are a variety of factions within the Mehsud tribe."
Military aircraft and artillery have been hitting Taliban targets in Waziristan for months, and it is unclear if and when ground forces will move in force.
As part of its campaign to weaken the Taliban, U.S. officials said the Pakistani army has been trying to negotiate with various factions, trying to split them off from harder-line groups loyal to Baitullah.
Several top members of his group, including one of his aides and the spokesman from the Swat valley, have also been captured in recent months.
The Pakistani army believes it has cleared nearly the entire former Taliban bastion in Swat valley with an offensive launched in April.
While largely forced out of Swat and Bajaur, there are still thousands of well-armed fighters in South Waziristan and other regions.