Islamabad - A suicide attack in the southwestern Pakistani city of Quetta on a paramilitary forces convoy has killed at least four bystanders, police say.

The attack aimed at vehicles carrying personnel of the Frontier Corps paramilitary force, which largely controls security in the restive province, in the Balochistan provincial capital on Saturday afternoon, said Abdul Razzaq Cheema, the city's police chief.

At least 15 people were wounded in the attack, he told reporters at the scene.

"Our investigation is ongoing, but from the evidence we have gathered from the scene so far, it was a suicide attack," he said. "It was a crowded area; there were rickshaws and other cars there at the time."

No group immediately claimed responsibility for the attack.

Security forces have been battling an armed, ethnic Baloch separatist movement in the province for almost a decade, and the region has also seen a wave of attacks by the Tehreek-e-Taliban Pakistan and its allies.

Increasingly, it has come under fire from local affiliates of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, such as the Lashkar-e-Jhangvi's Al-Alami faction.

Earlier this month, a senior police official was killed in a similar suicide attack on his vehicle in the provincial capital.

In October, at least seven elite anti-terrorism police personnel were killed in a roadside bombing aiming at a truck in which they were travelling.

Balochistan Home Minister Sarfaraz Bugti condemned the latest attack in brief remarks to the press on Saturday.

"Quetta is in a state of war," he said.

Additional reporting by Saadullah Akhtar in Quetta.