Sarfaraz Bugti, home minister for Baluchistan province, said hundreds of worshippers were attending services at the church in Quetta city ahead of Christmas.
As per latest report, there were at least 400 people present in the church, mostly women and children.
It later emerged that one of the attackers had detonated a suicide vest inside the church's premises. Two other fled the scene as the personnel tightened noose on them.
The security forces are conducting an operation to look for terrorists, he added.
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"We have cleared the immediate area around the church, and we are now clearing a peripheral area further out", Ansari said, speaking to the media at the site of the attack. Pakistan's resolve against terrorism can not be deterred by these cowardly acts.
The city administration has enforced an emergency in Civil Hospital and other medical institutions of the provincial capital.
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.
More than 200 people were gathered at the church for the service at the time of the attack.
This is a developing story and will be updated shortly.