A bomb exploded in the jittery southern Philippines but caused no injuries on Wednesday, amid warnings of more terrorist attacks a day after suspected Al-Qaeda-linked militants detonated an explosive that killed six people.
It was the fourth bomb in the region in two days. Three of them exploded, while a fourth was defused.
The latest explosive device -- an 81 mm mortar shell -- was found in front of a store and a bank in southern Cotabato city, said police Senior Inspector Samson Obatay.
He said a saleswoman noticed a man dropping off a suspicious package, picked it up and threw it away before it went off.
There were no casualties. Obatay blamed Al-Qaeda-linked militants, saying the bomb was similar to an explosion that killed six people and wounded 29 on late on Tuesday in Makilala town, 100 kms east in North Cotabato province and 950 kms southeast of Manila.
Another 81 mm device was safely defused in Makilala on Wednesday, officials said.
Colonel Ruperto Pabustan, the army brigade commander, said the defused device was attached to a cell phone and placed in a bag.
It too bore the hallmarks of the Al-Qaeda-linked Jemaah Islamiyah and Abu Sayyaf groups, he said.
The violence came hours after the US, Australian and British embassies issued warnings against travel to Mindanao and said the threat of imminent attacks or kidnappings was high across the archipelago, specifically in places frequented by foreigners.
Security officials said the bombs may have been linked to last week's arrest of Istiada Binti Oemar Sovie, the wife of Dulmatin, one of Asia's most wanted terror suspects for his alleged role in the 2002 Bali bombings that killed 202 people.
He and another Indonesian, Umar Patek, are believed to be hiding on the southern island of Jolo with Abu Sayyaf guerrillas.
President Gloria Macapagal Arroyo, a staunch US ally, promised the government "will make sure the perpetrators are hunted down and brought to justice".
She also instructed officials to contact the main Muslim rebel group, the Moro Islamic Liberation Front, to assist in the identification and arrest of the bombers.
The rebels, who are engaged in peace talks with the government, have denied any terrorism links, but there were concerns some guerrilla elements may be cooperating with militants.
Pabustan said the cell phone attached to the defused bomb, which was found about 75 meters from the site of Tuesday's explosion, contained a text message in the Maguindanao dialect, apparently directed at the bombers.
It read: "Where are you? You are the only ones who have not yet detonated (the bomb)".
Pabustan said the bomb also was planted on Tuesday but had failed to explode.
"We have not arrested anyone but we are really blaming JI for this," he said.
North Cotabato provincial police chief Federico Dulay said security forces across Mindanao, home to a decades-old Muslim insurgency, were on alert for more bomb attacks as several towns were holding festivities expected to attract large crowds.
In Kidapawan, the capital of North Cotabato, authorities prepared for a fruit festival on Wednesday.
In southern Zamboanga city, police were on alert to safeguard an annual Roman Catholic festival that culminates on Thursday with a public parade.