(CNN) Philippines President Rodrigo Duterte has visited the site of a deadly attack on a church in the southern Philippines which killed 20 and left scores injured.

The premier visited the town of Jolo for a few hours Monday afternoon, touring the site of the attack, meeting with civilian and military victims and attending a memorial for those killed in the attack, according to Philippines state media outlet PNA.

Sunday's bombing was claimed by ISIS raising fears of an increase in violence in the country's restive Mindanao region, following a referendum last week in which an overwhelming majority of voters backed self-rule

Monday afternoon's visit by the country's leader came as authorities put the southern Philippines city of Jolo on lockdown to ensure security and investigate the attack, according to CNN affiliate CNN Philippines.

Bombs exploded at the Jolo Catholic Cathedral during Sunday mass, officials said, killing 20 and injuring at least 81 people, including 14 soldiers and two police officers.

The first device went off inside the cathedral and the second targeted nearby soldiers who rushed to help the victims of the first explosion, a military spokesman said.

ISIS presence reaffirmed?

Mindanao, a region in the far south of the Philippines at the borders of Malaysia and Indonesia, has long been plagued by terrorism and unrest.

It is home to several Islamist insurgent groups, including Abu Sayyaf, which has been blamed for a number of attacks on civilians and Philippine government troops, as well as the kidnapping of several foreign nationals.

Supporters of last week's referendum, which will see greater autonomy granted to Muslim-dominated parts of Mindanao, hope it can bring a peaceful resolution to the protracted conflict, which has claimed more than 120,000 lives since the 1970s.

Soldiers and police cordon off the area after two bombs exploded outside the church.

But while Islamist and criminal groups have been active in the lawless tri-border area between the Philippines, Malaysia and Indonesia for years, the rise of ISIS-affiliated groups has led to a sharp escalation in violence.

In 2017, ISIS-affiliated militants laid siege to the city of Marawi in Mindanao for five months. The ensuing violence forced more than 350,000 residents to flee the city and the surrounding areas, as their homes were reduced to rubble by government airstrikes.

In the 150 days of the Philippine army operation to flush the militants out, more than 800 militants and 162 members of the government security forces were killed.

Analysts say Sunday's cathedral bombing attack had all the hallmarks of ISIS, which has been all but displaced from its former strongholds in Iraq and Syria, and shows that the group is still influential in the Mindanao region.

Speaking to CNN, Sidney Jones, director of Jakarta-based Institute for Policy Analysis of Conflict, said ISIS is "still active" in the region but has changed tactics since its 2017 occupation of Marawi.

Policemen and soldiers are seen in front of the Catholic Church where the bombing took place.

"The concern from the Marawi siege onwards had been that once Marawi ended, ISIS components would use violence in other places," she said. "We've had incidents in Lanao del Sur, Basilan -- the Lamitan bombing -- Cotabato and others since then."

Jones said the bombing shows the willingness of Islamist extremist elements in the region, many of which have pledged allegiance to ISIS, to mount attacks even though ISIS in the Middle East has suffered multiple defeats.

"I think it's a reminder that the establishment of the (autonomous region) does not eliminate extremism," Jones said.

"These guys march to a different drummer, they're not motivated by the establishment of the (autonomous region), they do not see an ethnic Maguindanaon-led political entity as the goal they have been striving for," she said, referring to the ethnic group which stands to gain autonomy under the proposed devolution of powers.

Local, national and international figures have condemned the attack, including the Moro Islamic Liberation Front (MILF), a key stakeholder in the peace process.

Leaders of the MILF, a longtime separatist movement which has embraced dialogue with the government, condemned the bombing and said they would help bring the perpetrators to justice.

Mohagher Iqbal, chair of the MILF peace panel, said his organization "is ready to support efforts in the apprehension of the perpetrators of the senseless violence that occurred in a place of worship while people were attending the morning mass," according to Philippine state media PNA.

A policeman carries a ballot box at a voting precinct in Cotabato on the southern Philippine island of Mindanao on January 21, 2019, during a vote on giving the nation's Muslim minority greater control over the region.

Threat level upped 'nationwide'

Fears are now rising that the bombing may trigger deadly attacks in other cities across the region. On Monday, Philippine National Police were placed on "nationwide high alert" following the recent bout of violence.

"It could show that other areas can also implement their own bombing attacks. Other groups could mount bombings in Cotabato, Davao, (and) other major cities in Mindanao," said Rommel Banlaoi, chairman of the Philippine Institute for Peace, Violence and Terrorism Research.

The attack demonstrates that "terrorist threats in Mindanao continue to be real," he added.

Almost two years after the Marawi siege brought the issue of Islamic extremism to the fore, parts of Mindanao remain under martial law, which is not due to expire until the end of this year.

The fact that a major attack could be mounted during martial law was a "major blow" to the government's policy, Banlaoi said.

"This mass casualty bombing occurred despite martial law and during a high security environment (in place for the plebiscite)," he said. "Let's see if it will mean an extension of martial law."