The blast hit a crowd of worshippers participating in a ceremony at the shrine of Sufi saint Shah Noorani in Khuzdar district, some 760 kilometres (472 miles) south of provincial capital Quetta; AFP reported.
"At least 43 people have been killed and dozens of others wounded," Sarfraz Bugti told a press conference in the southwestern port town of Gwadar, without providing a precise figure for those injured.
A senior police official and a security official confirmed the death toll.
Officials earlier said 25 people had been killed and at least 35 others wounded.
Hakim Lasi, a rescue official with the Edhi Foundation, Pakistan's largest welfare organisation, told Geo TV channel the charity had received reports via wireless radio saying that nearly 100 others were wounded, including women and children.
Local officials said worshippers were taking part in a devotional dance session, which is held daily before dusk, when the blast occurred.
Rescuers were scrambling to reach the shrine, which is located in a remote, mountainous region with limited medical facilities.
Authorities have dispatched ambulances and medical workers from Karachi, a three-hour drive from the blast site.
Up to 600 people were at the shrine at the time of the attack, according to local official Tariq Mengal, who told Geo TV that many devotees travelled to the site from Karachi during weekends.
Islamic State (IS/ISIL/ISIS) group claimed responsibility for the attack via Amaq, its affiliated news agency.
"35 dead and 95 wounded Shiite visitors in a martyrdom operation attack by the Islamic State fighter that targeted a shrine in a city in Balochistan," the agency said.
The bombing follows the killing of Amjad Sabri, a renowned Sufi singer, by two gunmen in Karachi in June.
Some observers have said that Sabri may have been assassinated because he was a high-profile Sufi.
Sufism, a mystic Islamic order that believes in living saints, worships through music, and is viewed as heretical by some hardline groups including the Taliban.
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