At least four people were killed and 20 others seriously injured when a suicide bomber detonated himself inside a church in Pakistan's Quetta city on Sunday, police said.

Four suicide attackers attacked the Bethel Memorial Methodist Church while a service was on, Balochistan Home Minister Sarfaraz Bugti told Dawn News.

One of the attackers detonated a suicide vest inside the church premises. Another was shot dead near the entrance by security forces after an intense gunfight, Bugti said. The other two terrorists escaped.

According to Bugti, around 500 people were present in the church during the attack. No group has claimed responsibility for the killings.