Through a "warden notice" posted on its website, the embassy said US citizens should avoid travel to Mindanao.
The Australian government, for its part, advised its citizens to exercise a "high degree of caution because of the high threat of terrorist attack," saying some reports suggest Manila could also be a target.
The warnings came just hours ahead of the bombing of a public gathering where 12 people were killed and 42 others wounded in Makilala, North Cotabato last night.
Local officials led by town mayor Onofre Respicio said the bomb was placed along a path leading to the town plaza where a public gathering was being held in celebration of the town’s 52nd founding anniversary.
Officials said the bomb was a mortar projectile, rigged with a time-delayed and battery-operated blasting mechanism.
US Embassy officials confirmed posting the "warden notice" to its citizens but did not specify the source of the threat.
"Although there is no specific targeting information, US citizens should avoid travel to that area," the embassy said in its notice, referring to Mindanao.
Australia also warned its citizens yesterday to avoid parts of the Philippines, citing reports that indicated "imminent" attacks on places frequented by foreigners.
Australia’s foreign affairs department, re-issuing a travel warning first given in June, said the threat applied to Mindanao, particularly the Zamboanga peninsula and the Sulu archipelago.
"Recent credible reporting indicates terrorists may be in the advanced stages of planning to attack places frequented by foreigners," it said.
"These reports indicate that attacks are imminent and could occur at any time," the department said on its website, repeating the terms of previous travel advisories for the region.
"Possible targets include restaurants, shopping malls including mega malls, hotels, hostels, guesthouses, bars or clubs in major population centres."
The US government, through its notice, advised its citizens working and living in Mindanao "to keep a low profile, avoid venues that cater to westerners and avoid public gathering places to the extent possible."
Last December, the American mission decided to temporarily close its services and beef up security around its premises citing "plausible" threat information.
The embassy, though, declined to disclose the nature of the threat that forced them to suspend services indefinitely.
The warnings came as US and Philippine officials admitted receiving credible intelligence that a terrorist group may be planning to carry out bombings in southern Mindanao, following the capture of the wife of a top Indonesian militant last week.
Istiada Binti Oemar Sovie was arrested in Jolo, Sulu last Oct. 3. She is the wife of Dulmatin, an al-Qaeda-linked Indonesian militant who is among Asia’s most wanted terrorist suspects, mainly for his alleged role in the 2002 Bali bombings that killed 202 people.
Philippine security officials monitoring the area said Dulmatin’s group and the Abu Sayyaf have long planned bombings in Mindanao.
They warned the capture of Dulmatin’s wife may expedite the plans in retaliation for her arrest. The officials requested anonymity because they are not authorized to speak to the media.
Earlier yesterday, a bomb also wounded at least six people in a public market in Tacurong City, Sultan Kudarat.
A security guard found the bomb, stashed in a bag filled with packs of corn chips, and hurled it away from a crowd before it exploded, preventing more casualties, Army Col. Felipe Tabas said.
No group claimed responsibility but Tabas said it a clear act of terrorism. "Nobody can do this except terrorists," he said.
Security at the market had been bolstered because of intelligence reports it could be targeted. About 10 security guards were deployed there recently, Tabas said.
North Cotabato Gov. Emmanuel Piñol, on the other hand, said he had ordered the provincial police to conduct an investigation over the bombing in Makilala.
Col. Ruperto Pabustan, commander of the Army’s 602nd
Brigade, said intelligence operatives have been dispatched to Makilala to help the police gather additional information to identify the bombers.
No group has claimed responsibility for the incident.
In Zamboanga City, police have been placed on alert to safeguard an annual Roman Catholic festival this week that culminates with a public parade on Thursday.
Zamboanga has been hit by deadly bomb attacks in recent years that were blamed on the Abu Sayyaf, which is on a US list of terrorist groups.
The United States has offered a $10-million reward for the capture of Dulmatin and $1 million for another Indonesian, Umar Patek.
The two Indonesians, wanted for the Bali bombings which killed a total of 92 Australians, have been given refuge by the Abu Sayyaf.