Four security men were martyred and 19 injured when a suicide bomber rammed his motorbike into a vehicle of the security forces on Mastung Road near Quetta on Sunday morning, reported 24NewsHD TV channel.
Seventeen of the 19 injured were members of the security forces whereas two others were civilians.
Outlawed Pakistan Tehreek-e-Taliban (TTP) has claimed responsibility of the suicide attack.
According to a spokesman of the Counter Terrorism Department, the suicide attack happened near Sona Khan Checkpost on Mastung Road and the security forces have cordoned off the blast site, not letting anyone in.
Police and Rescue teams have also reached the spot and have started to collect evidence from the blast site. They took into custody the severed head and other body parts of the suicide bomber.
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DIG Quetta Azhar Akram said at least 20 people were injured but their number may rise.
Eight of the injured people were shifted to Shaikh Zayed Hospital for treatment where doctors described their condition as very serious.
According to AFP, the bomber targeted Frontier Constabulary guards in the Mian Ghundi neighbourhood of Quetta -- around 140 kilometers from the frontier with Afghanistan -- where Hazara merchants were trading vegetables.
Three died immediately in the blast, with another officer dying later of his wounds, said Azhar Akram, a deputy inspector general of police.
Akram told AFP that 17 guards and two civilians were wounded in the blast. Three are in a critical condition, he said.
A spokesman for the police's Counter-Terrorism Department confirmed the attack.
Prime Minister Imran Khan, PPP Chairman Bilawal Bhutto Zardari, Co-Chairman Asif Ali Zardari, PML-N President Shehbaz Sharif have in separate statements strongly condemned the TTP terrorist attack on the FC checkpost.
Asif Zardari said the continued terrorist attacks were the result of the non-implementation of the National Action Plan (NAP).
Reporter Rizwan Saeed