At least four people have been killed and more than two dozen injured in an explosion outside a police station in southwest Pakistan, an official said.

The blast on Tuesday evening, which apparently targeted the police, took place in the car park of the city police station in Quetta, the capital of Balochistan province. It was the latest in a series of similar attacks.

Provincial Home Minister Mir Zia Langov told Anadolu Agency - a news service - that four people were killed and 26 injured, including many policemen in the attack.

However, Abdul Razzaq Cheema, the Quetta police chief, told Reuters news agency that five people, including two policemen, had been killed. He added that two women and a child were among the 27 injured.

Cheema said an explosive-rigged motorcycle was detonated to target a police van parked outside the station in the heart of the city's busy shopping area.

Footage aired on local broadcaster Dawn News showed ambulances carrying the dead and wounded to hospital. Blood-soaked caps, shoes and other belongings were scattered around the scene among pieces of broken glass and wooden doors.

There was no immediate claim of responsibility for the attack.

Pakistan Taliban and Baloch separatists have for long been targeting security forces in the region.

In a similar blast last Tuesday, two people were killed and 16 more injured when an explosive-rigged motorbike went off outside a shop in Quetta.

Violence in the Balochistan province, which borders Afghanistan and Iran, has raised concerns about the security of investments, in particular, an energy link planned to run from western China to Pakistan's southern port of Gwadar.