An explosion targeting a police vehicle in Pakistan's southwestern Quetta city on Friday killed at least 13 people and injured around 20 others, officials said.

The blast, in front of the office of the Quetta police chief, was claimed by both the local affiliate of the Islamic State group and by Jamaat-ul-Ahrar (JuA), an offshoot of the Pakistani Taliban, according to the SITE monitoring group.

There was no immediate explanation for the dual claims. Islamic State Khorasan Province, the Middle Eastern group's affiliate in Afghanistan and Pakistan, has been known to work with the myriad of Pakistani militant groups in previous attacks, including with JuA.

Friday's attack in Quetta, the capital of insurgency-wracked Balochistan province, "targeted a police pick-up in front of the IG (Inspector General) office", said senior police official Mohammed Tariq.

"A motorcycle was also destroyed in the explosion," he said.

"The death toll has climbed to 13," Fareed Ahmed, medical superintendent at Civil Hospital told AFP, with around 20 injured, most by shrapnel. Police officials said nine policemen were among the dead.

At the hospital, worried children stood by the bloodstained cots of wounded relatives, and Pakistani soldiers visited injured colleagues.

Stunned survivors could give few details about the attack. "I was sitting on a chair. There was an explosion. I got injured and fell down," said one victim, Gulzar Ahmad.

Pakistan has been battling Islamist and nationalist insurgencies in mineral-rich Balochistan, the country's most restive province, since 2004, with hundreds of soldiers and militants killed in the fighting.

Bordering Iran and Afghanistan, it is the largest of Pakistan's four provinces, but its roughly seven million inhabitants have long complained they do not receive a fair share of its gas and mineral wealth.

A greater push towards peace and development by Pakistani authorities has reduced the violence considerably in recent years.

The push includes starting work on a massive Chinese infrastructure project -- the China-Pakistan Economic Corridor -- which gives Beijing a route to the Arabian Sea through Balochistan's deep sea port of Gwadar.

Beijing is ramping up investment in its South Asian neighbour as part of a plan unveiled in 2015 that will link its far-western Xinjiang region to Gwadar port with a series of infrastructure, power and transport upgrades.