At least six people have been killed and 29 wounded in a powerful bomb blast in the centre of the southern Philippines city of Cotabato, police have said.

The explosive device was placed in a vehicle - either a motorcycle or a van - parked near a hospital and a school, city police chief Senior Superintendent Rolen Balquin said on Monday.

"The explosion was on a very busy street," Balquin told local Catholic radio station DXMS.

The late-afternoon blast damaged at least four vehicles and triggered a fire that engulfed a nearby mortuary and a tyre repair shop, he added.

No group has claimed responsibility for the attack, and Balquin said the motive was still being investigated.

Mujiv Hataman, governor of a regional autonomous area whose office is 800m away from the blast site, said the explosion rattled his windows and shattered a quiet afternoon just before Muslims were to end their Ramadan fast.

"That area has always been a busy street, full of traffic. There are a lot of establishments there and people come and go at all hours," he told AFP news agency by telephone.

Muslim-Catholic city

Cotabato is a mixed Muslim-Catholic city on the main southern island of Mindanao, where Muslim fighters have waged a decades old rebellion that has left thousands dead.

It was the second bombing to hit Mindanao in 10 days.

A powerful blast at a restaurant packed with doctors and pharmaceutical salesmen left eight people dead on the Mindanao city of Cagayan de Oro on July 26.

The Cotabato explosion also came just shortly after the Bangsamoro Islamic Freedom Fighters, a breakaway rebel faction, warned of intensified attacks against the government targets.

In 2011 the BIFF split from the 12,000-strong Moro Islamic Liberation Front (MILF), which is negotiating peace with the government of President Benigno Aquino.

Source: Agencies