ZAMBOANGA, Philippines: A suspected bomb attack left six dead while a separate grenade blast wounded five as violence marred New Year celebrations in the Philippines, authorities said Wednesday.

A blast from what is believed to be an improvised bomb caused the deaths in the town of Sumisip on the volatile southern island of Basilan two hours before midnight Tuesday, police said.

Basilan is a known hotbed of the Abu Sayyaf, an Al Qaeda-linked group blamed for the country’s worst terrorist attacks.

Troops and police said the blast ripped through a party at the residence of Manuel Cisneros, a pro-government volunteer militiaman and driver for a nearby Catholic church.

Police said the motive for the attack was not clear, although Cisneros had apparently been helping protect rubber plantation workers from extortion by armed gangs including the Abu Sayyaf.

“We suspect it was the Abu Sayyaf,” provincial police chief Senior Superintendent Mario Dapilloza said over DZBB radio in Manila, describing the blast as “very powerful”.

However, investigators were also considering the possibility of an accidental explosion involving a grenade carried around by the militiaman.

The Abu Sayyaf, founded in the 1990s with seed money from Osama bin Laden, has been blamed for the country’s worst terrorist attacks, including kidnapping of foreigners and deadly bombings.

The government has said the group degenerated into a crime gang with no clear ideology over the years, and its ranks fell to just over a few hundred.

Last year Abu Sayyaf gunmen ambushed a convoy carrying plantation workers, killing six and wounding 22 in the same area.

Authorities said an Abu Sayyaf faction on a nearby island was believed still to hold a number of foreign as well as Filipino hostages, including two European birdwatchers and a Japanese treasure hunter.

In December a Jordanian TV reporter held in captivity for 18 months claimed to have escaped from his captors.

Elsewhere in the south Tuesday, five civilians were wounded when gunmen aboard a motorcycle lobbed a grenade outside a church in the town of Carmen on Mindanao island.

The victims were about to attend mass.

Local mayor Rogelio Talino said it was not clear whether they were targeted or whether it was a random attack.

Muslim rebels negotiating peace with the government as well as other armed factions are known to operate in the area, the police and military said.