MANILA ? (UPDATE 2) Muslim terrorists from the remote southern Philippines may have been behind a bus bomb attack in the nation's financial hub that killed five people, authorities said on Wednesday.

A mortar shell triggered by a mobile phone led to Tuesday's explosion that ripped apart a bus travelling along one of Manila's main roads, the city's police chief and President Benigno Aquino's national security adviser said.

"A Nokia cellphone is the device they used to trigger the explosion. It acts like a command-detonated explosive," the security adviser, Cesar Garcia, said on ABS-CBN television.

"The fact that... the device used was an improvised explosive device similar to the ones used by terrorist organizations in the southern Philippines raises the possibility it was a terrorist attack."

While Garcia said it was too early to say exactly who was behind the blast, he pointed out the attack was very similar to a bus bombing on the same road in Manila that killed four people and injured 36 others on Feb. 14, 2005.

"Investigation into the 2005 Valentine's Day bombing showed the suspects rode the bus, carried the (bomb) in a backpack, left the backpack, got off... (and) detonated the bomb with the use of a cellphone."

The Abu Sayyaf, a small group of Islamic militants blamed for the nation's worst terrorist attacks and a string of kidnappings, claimed credit for the 2005 attack, although it has remained silent following Tuesday's explosion.

Garcia emphasized that militant groups such as the Abu Sayyaf always wanted to attack Manila, which is more than 1,000 kilometers (600 miles) from their strongholds in remote jungles and isolated Muslim-populated towns in the south.

"Metro Manila has always been a long-term aspirational target of the organizations operating in the southern Philippines," Garcia said.

"It's been five years since a major terrorist attack in Manila. We have to be lucky all the time (to stop an attack). They have to be lucky just once."

Interior Secretary Jesse Robredo said that authorities had no reason to believe more attacks were coming, but that extra security measures had been put in place at bus and rail stations, as well as other key public places.

"There will be some extra inconveniences, but our public transport system will be safe," he said.

Aquino said on Tuesday after the attack that his government had been warned last year that unnamed Muslim militants had been planning to stage a bomb attack in Manila.

He said he did make the report public because his officials believed the militants did not have the capabilities to carry it out.

"We have had a report that there were terrorist groups planning the same (an attack). But the assessment at the time was that there was a lack of resources to be able to carry it out and a lack of support base," he said.

His admission came after he repeatedly denounced the US and five other Western governments in November last year when they issued travel advisories warning that a terrorist attack was imminent in Manila.

The southern Philippines has long been an area of conflict, with the Muslim population there seeking a state independent from the rest of the mainly Christian country.

The 12,000-strong Moro Islamic Liberation Front (MILF), which is set to restart peace talks shortly with the Philippine government, has waged a decades-old rebellion in the south that has claimed an estimated 150,000 lives.

The MILF has in the past denied any links with the Abu Sayyaf and foreign Islamic militants allegedly hiding out or training in Mindanao.

Robredo said the number of people killed in Tuesday's attack rose from four to five on Wednesday, with 14 people injured.

Media reports said the latest fatality was a 22-year-old female call center worker who died in hospital on Wednesday morning from head injuries sustained in the blast.

With reports from Maria Cristina Santos and TJ Burgonio, Philippine Daily Inquirer