Six people died and 13 others were wounded when a bomb ripped through a packed passenger bus in Digos City in Davao del Sur province Monday, police and other authorities said.
The blast, caused by an improvised explosive device, tore through the bus owned by Metro Shuttle company, which was parked inside a terminal after coming from Davao City. The Malita town-bound bus was unloading passengers when the bomb went off.
It was not immediately clear how many passengers were aboard the bus or how many people were at the terminal at the time of the explosion.
The identities of the fatalities, including a pregnant woman, and those wounded were unavailable as of posting time.
Gov. Douglas Cagas said the bus driver and a policeman who was a passenger were among those hurt in the blast.
Regional Army spokesman Maj. Armand Rico confirmed four were killed, adding that troops have been sent to the area to set up roadblocks along possible escape routes.
"It was so powerful that the blast decapitated the head of a civilian," Rico said.
Witnesses quoted by police said the vehicle's roof and sides were ripped open by the force of the explosion.
No one immediately claimed responsibility for the incident.
Officials had warned the public to brace for possible attacks by Muslim separatist rebels, who are on the run from a military offensive on Mindanao.
Troops are hunting down renegade guerrillas from the Moro Islamic Liberation Front (MILF), responsible for a wave of deadly attacks this month on Mindanao.
More than 100 rebels as well as more than 40 civilians and soldiers have been killed in four weeks of fighting.
The clashes have put on hold peace talks between the government of President Arroyo and the MILF, which recently warned that the upsurge in violence could lead to a full-blown ethnic war.
Police said they were also looking at the possibility the attack could have been carried out by a criminal gang, as the bus firm had apparently received threats.
Last month, another Metro Shuttle bus was bombed, also in Digos, killing three people and wounding a number of passengers.
That attack was blamed on the Al Khobar gang, which is composed mostly of foreign Muslim rebels and preys on businesses in the south.
An official from the bus firm last month said the gang demanded 50,000 pesos (about 1,000 dollars) a month in extortion money, threatening attacks if it was not paid. With Agence France-Presse and Michelle Robin, ABS-CBN Davao