The attack targeted Bethel Memorial Methodist Church as worshippers gathered inside to attend a Sunday midday service.
A suicide bomber detonated his explosives at the gate of the church, prompting a police operation, officials told.
A second attacker fired upon worshippers, before being killed by security forces at the scene.
“We have cleared the immediate area around the church, and we are now clearing a peripheral area further out,” Moazzam Jah Ansari, police chief of Balochistan province, told reporters at the site of the attack.
Witnesses reported a heavy exchange of gunfire in the neighbourhood as police worked to clear the area.
“People were fleeing to the corners [of the church]. I couldn’t understand what was happening; it happened so suddenly,” a woman, who was at the church when the attack occurred, said on condition of anonymity.
Waseem Ahmed, an official at the nearby Civil Hospital, said 33 people were wounded in the attack.
More than 200 people were gathered at the church for the service at the time of the attack.
The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group claimed responsibility for the attack in an online statement published by its Amaq outlet. The group did not provide any evidence for its claim.
Quetta, the capital of Balochistan province, has been at the centre of recent violence in Pakistan.
The city has come under attack both from armed groups allied with the Tehreek-e-Taliban Pakistan and separatist fighters.
Last month, a suicide attack targeting paramilitary soldiers killed at least four people and wounded 15 others.
Earlier that month, a senior police official was also killed in a similar attack, while in October at least seven police officials were killed in another roadside bombing.