Badruddin Haqqani died last Tuesday in North Waziristan, a relative said.
The death has not been confirmed by US or Pakistani officials.
The Haqqani network has carried out high-profile attacks against foreign troops in Afghanistan. If confirmed, the death would be a major victory for US forces, correspondents say.
In a separate development, Nato air strikes killed a regional commander of the Pakistani Taliban on Friday inside Afghanistan, officials said.
Mullah Dadullah was killed along with a dozen of his fighters in Afghanistan's eastern Kunar province. He had taken over as leader of the Taliban in Pakistan's Bajaur tribal area earlier this year.
Badruddin Haqqani has been described as a senior operational commander with the Haqqani network - masterminding and directing ambitious attacks on high-profile targets.
He was also responsible for training camps, and for extorting funds from contractors, the BBC's Orla Guerin in Islamabad says.
Tribesmen in North Waziristan and sources in the local administration told the BBC he is now dead and buried.
Local sources also told the BBC that his replacement had already been named. But experts think the death would affect operations by the Haqqani network, our correspondent says.
North Waziristan - where the group is based - is seen as a haven for Taliban and al-Qaeda militants.
A key commander in the Pakistan-based militant Haqqani network has been killed in a US drone strike, according to a family member and local sources.