MANILA A bomb exploded on a bus in the Philippine capital on Tuesday, killing four people and wounding 14, and officials said they could not rule out terrorism as they began an investigation into the blast.
Several rebel groups including communists and Islamists are battling security forces in various parts of the Philippines, a staunch U.S. ally, but attacks in Manila are rare.
The explosion happened at around 2 p.m. (1 a.m. EST) when the bus was on a highway in the main business district of Makati, as it was nearing a railway station.
"It's an IED, improvised explosive device," Nicanor Bartolome, head of the National Capital Region police, told reporters.
"The affected area is the sixth row, right side of the bus. Apparently the explosive was placed underneath the passenger seat, because the effect of the explosion was massive in that area."
President Benigno Aquino, who was due to speak at a hotel in Makati on Tuesday evening, said an investigation would determine whether the explosion was caused by a bomb and "does it ... point to a particular group that is fond of using that particular method of terrorizing the community".
The bombing came about three months after a number of foreign embassies had warned of a heightened risk of attacks in the Philippines, including the capital, which the government had said was not warranted.
Aquino said no country was immune from the terrorist threat, and security forces would review their risk assessments.
"There were reports that they may have plans, but the assessment is they lack the capability, they lack the resources, they lack the support from those who can help with safe houses, those who can help secure their materials. I've asked for a review how this happened," he told a media briefing.
"I have my suspicions but as president of the republic, I cannot engage in rumors; there has to be evidence," he said, adding he would not pressure investigators for quick answers.
Traffic on the northbound lanes of the EDSA highway came to a standstill as investigators searched for evidence, causing large delays across the metropolis that were set to worsen as city workers returned home. Trains were running as normal.
The explosion came six years after a similar incident in Manila that killed six people. Abu Sayyaf, a Muslim rebel group linked to Al Qaeda, had claimed responsibility for that blast and two other bombings in the south of the country on February 14, 2005, that killed 13 people and wounded more than 150.
Abu Sayyaf also claimed responsibility for a February 2004 bombing of a ferry near Manila Bay that killed more than 100 people, and bombings in Manila on December 30, 2000, that killed 22 people and wounded nearly 100.
Remote southern islands in the Philippines have become a sanctuary for camps and training bases of Southeast Asian Islamist militants aligned with the Abu Sayyaf and Jemaah Islamiah groups.
Intelligence reports say about 50 Indonesian, Malaysian and Singaporean militants have been hiding on the southern island of Mindanao and nearby islands of Basilan and Jolo since early 2000.
The Philippines has been trying to end two long-running insurgencies by Muslim separatists and Maoist rebels. Peace talks with both groups are due to restart next month.
Last year, security forces said a splinter group of Muslim separatists was behind a bus attack on Mindanao that killed 10 people. There have also been bombings in the south related to extortion gangs and political or clan disputes.
(Writing by John Mair; editing by Andrew Marshall)