A bomb exploded Monday at a crowded market killing 13 people in the latest deadly attack in southern Philippines, officials said.
About 26 others were injured in Koronadal city, said Police Chief Superintendent Danilo Posadas.
A second bomb was found two hours later and taken away by a police bomb squad.
Koronadal Mayor Fernando Miguel said the Muslim extremist Abu Sayyaf group, which is on the US list of terrorist organisations, claimed responsibility for the attack.
In a radio interview, Miguel said a man identifying himself as Abu Solaiman of the Abu Sayyaf group called him shortly after the blast, warning of "more bombings in the days to come".
The man has been calling since 2005 demanding four million pesos (70,000) monthly to "spare Koronadal city from bomb attacks", the mayor said, adding he had refused.
There was no independent confirmation that the call came from Solaiman, who has been linked to bombings in General Santos city farther south and is one of five Abu Sayyaf leaders with a 4.2m bounty offered by the US State Department for kidnappings and killings in the southern Philippines in 2001-02.
Posadas said an initial investigation indicated the bomb was fashioned from an 81 mm mortar shell. Police suspect the person who planted it died at the scene.