"Two of the wounded people died overnight in the hospital," said provincial home minister Zia Langove, adding that six people were still in a critical condition.
"The US condemns this heinous terrorist attack in a place of worship," Paul Jones, the US ambassador to Pakistan, said in a statement. "I extend our condolences to the families of those who lost their lives."
Mineral and gas-rich Baluchistan, of which Quetta is the capital, is at the center of the $60 billion China-Pakistan Economic Corridor, part of China's Belt and Road project.
But violence in the province has fueled concerns about the security of projects such as a planned energy link from western China to Pakistan's southern port of Gwadar.
The attack was the second in the city this week, while police said they had foiled another by killing a suicide attacker in Rawalpindi city near Islamabad after he shot and killed two policemen.
The Taliban denied in a statement that some of its members, including a senior commander, were killed.
Local officials in Quetta's police and district administration would not confirm whether the Dar-ul-Aloom Shariah seminary belonged to the Afghan Taliban under a state policy which denies the presence of the group on its soil.
However, two officials said on condition of anonymity that the seminary was part of the Afghan Taliban.
Baluchistan has faced an insurgency by separatist and nationalist groups who want a greater share in revenues earned from the local resources.
Islamist militants, including Islamic State which consists of splinters from local Taliban and sectarian groups, also have a strong presence in the region.