"The next bombings will be in Zamboanga City and Basilan," said a cell phone text message sent by self-proclaimed spokesman Abu Omar to the RGMA network.
The warning came just two days after a powerful Abu Sayyaf bomb explosion ripped through a two-storey convenience store in Jolo island, killing 9 people and wounding more than two dozens.
Security officials appealed to the public to stay calm and be vigilant, saying, authorities were hunting down members of the Abu Sayyaf group, blamed for the series of terrorism and kidnappings for ransom in the southern region.
"We urge the public to cooperate with authorities and report to us any suspicious persons or abandoned package. Do not listen to rumors, but stay vigilant,"Air Force Major Gamal Hayudini, spokesman for the military's Southern Command, told the Zamboanga Journal.
He said Omar had been sending threat letters in the past to different radio and television stations in Zamboanga City, but his real identity remains unknown. Omar also previously threatened to kidnap and kill local journalists who criticized the Abu Sayyaf group.
It was not immediately known if Omar also claimed responsibility for the attack, but the Southern Command tagged Abu Sayyaf militant Ismin Sahiron as behind the bombing in Jolo.
Sahiron is the son of Radulan Sahiron, a senior Abu Sayyaf leader in Jolo and wanted by the United States for terrorism, said Hayudini.
The U.S. Department of Treasury has recently designated the elder Sahiron and two other Abu Sayyaf leaders Jainal Antel Sali Jr. and Isnilon Totoni Hapilon, a move that freezes any assets they may have under American jurisdiction.
"The Abu Sayyaf Group instills terror throughout Southeast Asia through kidnappings, bombings and brutal killings. This action financially isolates senior members of the ASG, who have planned and carried out vicious attacks on Americans, Filipinos and innocent citizens from around the world," said Patrick O'Brien, the Treasury's assistant secretary for terrorist financing and financial crime.
Southern Command chief Major General Gabriel Habacon has ordered a tightened security in the region following the bombing in Jolo.
The U.S. also deplored the bombing in Jolo and said it will continue to work closely with the Philippines to fight the threats of terrorism.
“We deplore the targeting and killing of innocent civilians. With the Armed Forces of the Philippines, we are committed to continue to provide humanitarian assistance to the people of the Sulu region."
President Gloria Arroyo and Mindanao Senator Aquilino Pimentel condemned the latest attack. "I condemn in the strongest terms this most recent attack in Sulu.
"Once more, and with a deep sense of urgency, I ask Congress to pass the anti-terrorism law that will enable our nation to constrict, contain and control this threat more effectively," she said.
The senator feared the bombing was aimed at sabotaging the peace process in Mindanao. "Saboteurs of peace in Sulu are killing innocent people to promote their own ends. It's very unusual incidence that it's budget time and the desire for more appropriations money could be a motive," he said.
Pimentel said the timing of the bombing was "very unusual" as it happened while the Senate is set to take up the budget of the Department of National Defense and Armed Forces of the Philippines. He did not elaborate.
Brigadier General Francisco Callelero, an army spokesman, said the Southern Command was investigating reports the attack was connected to a failed extortion by the Abu Sayyaf group.
"Our investigators found a letter demanding money from the managers of the Sulu Cooperative Store days before the attack," he told reporters in Zamboanga City.
The Abu Sayyaf is on a US list of terrorist organizations and Washington has offered as much as $10 million bounty for the capture of the group's chieftain Khadaffy Janjalani and other known leaders.