According to Pakistan Today, more than 20 people have been wounded in the blast, which took place after Friday prayers.
Taliban leader Haibatullah Akhundzada was not in the mosque when the bomb went off but his younger brother, Hafiz Ahmadullah, was among those killed.
The killing of the brother of Taliban leader comes at a crucial time when talks between rebels and the United States have entered their final phase.
The report says that no group has claimed responsibility for the blast.
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The mosque was known to be visited by members of the Afghan Taliban, the sources said. An emergency has been declared at the Civil Hospital, Quetta and the injured have been shifted to nearby hospitals.
Abdul Ali, a local police chief, said the bomb was planted inside the mosque and was remotely detonated. Militants from both the Pakistani Taliban and the extremist Islamic State militia have targeted places of worship in Balochistan in the past.
Balochistan -long been plagued by an insurgency and targetted killings - is reeling under a fresh wave of terrorism, as Baloch insurgents and other religious outfits have ratcheted up attacks in the restive province, targetting LEAs and minority Shias, respectively. But Friday's blast will raise concerns about prospects for peace.
Balochistan is also key to the China-Pakistan Economic Corridor (CPEC), part of Beijing's Belt and Road initiative.