It remains unclear whether Mansour survived the gunbattle, which threatens to derail a renewed regional push to jump-start peace talks with the Taliban.
The clash broke out just four months after Mansour was appointed Taliban leader in a deeply acrimonious leadership succession. Talks had stalled after the militants belatedly confirmed longtime leader Mullah Omar's death in late July.
The Taliban officially rejected as "absolutely baseless" the reports of the gunfight, which officials and militant sources said was triggered by a verbal duel at an insurgent gathering that cascaded out of control.
"We are trying to establish whether Mansour is dead or alive," said Sultan Faizi, the spokesman for the Afghan first vice president, AFP reported Thursday.
A senior Pakistani intelligence official told AFP that Mansour had been "very seriously injured" in what he described as a "heavy exchange of fire" at a gathering of militant commanders.
An Afghan intelligence official and multiple insurgent sources close to Mansour's group confirmed the account, adding that the firefight left at least four Taliban members dead and several others wounded.
"The reports are still sketchy, but the sheer volume of rumors suggesting that something has happened to Mansour will pressure the Taliban to offer proof that he's alive," a Western official in Kabul said.
"Simply posting denials on their website won't be considered credible enough, especially after Mullah Omar's death was concealed for years."
The purported firefight exposes deepening divisions within the fractious militant movement, which saw its first formal split last month after a breakaway faction surfaced.
There was conflicting information on the location of the confrontation, with some sources claiming that it took place near the southern Afghan city of Kandahar, the spiritual birthplace of the Taliban.
But they all agreed the meeting was at the home of Abdullah Sarhadi, a commander in Mansour's group and a former Guantanamo Bay detainee.
"There were differences on some points which later turned into harsh words, then Sarhadi opened fire and the others returned fire," a Taliban source said.
It was unclear what the argument was about.