MANILA, Jan. 29 (Xinhua) -- Philippine security forces on Tuesday stepped up the manhunt for the suspects that carried out Sunday's twin bombings at a Roman Catholic Cathedral in the southern Philippine Sulu province that killed 21 people and wounded more than 100 others.

The military launched ground assault and airstrikes against the Abu Sayyaf militants in Palitkul town in Sulu, the lair of the extremist Abu Sayyaf.

Philippine President Rodrigo Duterte earlier had ordered the military to pulverize and launch an all-out war against the Abu Sayyaf. On Tuesday, Duterte told reporters that he ordered the military to "arrest the killers" and to "go into a punitive action" against the Abu Sayyaf.

"I order you to destroy the organization. I order you now, crush the Abu Sayyaf by whatever means," Duterte told the military on Monday when he visited Jolo, where the church is located.

Defense Secretary Delfin Lorenzana echoed Duterte's assessment, saying the second bomb that exploded at the church's entrance may have been carried out by a suicide bomber.

Lorenzana told Xinhua in an interview that the Abu Sayyaf militants "affiliated with the ISIS" were behind the "terrorist attack."

He said that security was tightened in Basilan, Sulu and Zamboanga City as early as August last year when the military intercepted reports that Abu Sayyaf extremists were planning to launch bombings targeting churches.

"And so since then these churches were guarded by soldiers, everyday, 24 hours a day. But for some reasons two bombers still (managed to enter Jolo to bomb the cathedral.) They are terrorists," Lorenzana said.

Senior Superintendent Bernard Banac, the spokesman for the Philippine National Police (PNP), said there are ongoing police operations to hunt the suspects, adding that all angles are being looked at to ensure that the perpetrators are arrested.

"There are leads that investigators are looking into including surveillance camera footage of possible suspects seen in the vicinity of the cathedral moment before the explosions," Banac told a news conference.

Banac said the investigation on the suspects is ongoing. "Right now we are in the process of verifying the identities of these persons as seen in the CCTV footage," he added, referring to the closed-circuit television footage.

He said security operations to and from Jolo have been intensified, and that roadblocks and checkpoints have been set up in and around Jolo.

Authorities also sent more elite troops and policemen to the province to maintain peace and help in the ongoing operations to hunt the suspects.

Authorities are piecing together what led to the deadly blasts that occurred amid tight security.

Police Senior Superintendent, Sulu Provincial Police Office Director Pablo Labra, however, said nothing is conclusive about the theory that the attack was carried out by a suicide bomber. "We cannot prove that right now," he told a local television in an interview.

Labra added that witnesses interviewed by police claimed a person left the bag or a package in one of the pews in the middle of the church.

Police bomb experts are now in Jolo, sifting through the debris inside and outside the cathedral for fragments that will help reconstruct the explosive device and identify the chemical composition of the explosive material used.

"This is to ensure that there are no bombs remaining in the area," Banac told reporters, stressing the need to put Jolo on a lockdown.

"These are precautionary measures that while police (bomb) experts are conducting a sweep, no people are coming in and out of the site. This is for the safety of all," Banac said.

Col. Gerry Besana, spokesman of the Armed Forces of the Philippines' Western Mindanao Command (WestMinCom), released the footage showing the alleged suspects walking in the street in the vicinity of the cathedral.

He said one of the suspects, caught by a street surveillance camera, was the brother of a slain Abu Sayyaf leader.

The WestMinCom and the PNP said the suspect was seen in the CCTV footage handing something to a companion before running away.

Besana said his motive was to avenge the death of his brother who was killed by police in August last year.

PNP Chief Director General Oscar Albayalde said Kamah is a known member of the so-called Abu Sayyaf sub-group "Ajang-Ajang."

Jolo is the base for Abu Sayyaf militant group. The gang has recently become better known for criminal brutality, kidnaping-for-ransom activities, bombings, and beheadings in western Mindanao, particularly in their lair in the island provinces of Basilan and Sulu.

The AFP created last month an Army infantry division that will mainly hunt down the elusive Abu Sayyaf terrorists in the southern Philippine Sulu province in a bid to eliminate the threat posed by the Abu Sayyaf terrorists.

The Jolo attack took place while Mindanao is under martial rule.