A powerful bomb which was heard for kilometres around killed six people Monday in a southern Philippine city where the United States and other governments earlier warned of a terror threat.
The explosive device was placed on a vehicle -- either a motorcycle or a van -- parked on a busy road near a hospital during the afternoon rush hour, said city police chief Senior Superintendent Rolen Balquin.
Cotabato photographer Mark Navales described a blood-spattered road strewn with charred bodies and smouldering vehicle parts.
"I saw three mangled human remains, one of them burnt," he told AFP.
"I have never before seen (the effects of) such a powerful blast in Cotabato."
It was the second bombing to hit Mindanao island in 10 days.
A powerful blast at a restaurant packed with doctors and pharmaceutical salesmen left eight people dead in the city of Cagayan de Oro on July 26.
It was not immediately clear whether the two bombings were linked.
At least 26 others were wounded in Monday's attack, although no group immediately claimed responsibility, Balquin said.
Those killed were motorists passing along Sinsuat Street, the main highway in Cotabato that is lined by commercial establishments.
Among the injured were police officers in a vehicle passing through the area, he said.
The bomb went off as a bulletproof car carrying city administrator Cynthia Gulani-Sayadi was passing by. She escaped unhurt, but two bodyguards following her in a separate van were among the dead.
The attack came a month after the United States, Australia and Canada warned its diplomatic staff against travelling to Cotabato and two other southern cities on Mindanao -- Zamboanga and Davao -- over fresh threats of terrorism.
The nature of the threat was not specified but related to "terrorist and insurgent activities", the US embassy had said.
Muslim insurgents have waged a decades-old rebellion on the island that has claimed an estimated 150,000 lives, but the main rebel group is now talking peace with the government.
In Manila national police spokesman Senior Superintendent Reuben Thedore Sindac said investigators were looking at various possible motives, including rivalry among local politicians.
He said it was too early to say whether Sayadi was the target, but it was among the angles being considered.
Asked whether the attack could have been linked to the US terror warning, Sindac said he did not want to speculate.
Cotabato is a mixed Muslim-Catholic city on Mindanao that had been attacked by militants in the past, and where other armed groups, including kidnappers, operate.
Mujiv Hataman, governor of a regional autonomous area whose office is 800 metres (2,625 feet) 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.
Ramadan is officially set to end on Friday for the minority Muslims in the largely Catholic country of 100 million.
"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 by telephone.
"The blast was heard kilometres away. It was powerful," he said.
The late-afternoon explosion damaged at least four vehicles and triggered a fire that engulfed a nearby mortuary and a tyre repair shop, he added.
The Cotabato explosion also came just shortly after the Bangsamoro Islamic Freedom Fighters, a breakaway rebel faction, warned of intensified attacks against government targets.
The group split in 2011 split from the 12,000-strong Moro Islamic Liberation Front (MILF), which is negotiating peace.
Three of its guerrillas were killed last week near Cotabato in clashes after they fired on an oil tanker and laid two roadside bombs.