Philippine officials say a powerful explosion has killed four people and injured at least 15 others on board a passenger bus in the capital, Manila.
Metro Manila police chief Nicanor Bartolome said Tuesday that an explosive device appeared to have been placed beneath a passenger seat in the center of the bus.
The blast, which was powerful enough to damage a nearby concrete wall, occurred at the edge of Manila's Makati business district.
President Benigno Aquino acknowledged the government had received intelligence reports last year indicating terrorist groups were planning attacks in Manila. However, he said, intelligence officials did not believe the groups had enough resources to carry them out.
President Aquino said he has instructed his security team to reassess the terror threat level.
Tuesday's incident came about three months after the United States and other nations issued travel advisories to their citizens warning of a potential terrorist attack in the Philippines, including Manila.
The Pacific nation has been plagued for decades by terrorist attacks carried out by communist and radical Islamic insurgents.
Abu Sayyaf, a group linked to al-Qaida, claimed responsibility for the 2004 bombing of a ferry near Manila Bay that killed more than 100 people. A group of Muslim separatists were blamed for a bus attack last October that killed at least nine people in the southern Philippines.
Attacks are also carried out by armed gangs involved in extortion.
Some information for this report was provided by AP and AFP.