Law enforcers conduct an investigation at the site of an explosion in Marawi city, Philippines on Sunday. AP

Philippine police have identified at least two suspects in the bombing of a Catholic Mass that killed four people, a regional police chief said on Monday, vowing to hunt down those behind the blast, which was claimed by Daesh militants.

The bomb went off on Sunday during a service at a university gymnasium in Marawi, a city left in ruins in 2017 by a five-month military campaign to end a bloody occupation by Daesh loyalists that had triggered alarm across Asia.

"(We have persons) of interest, but the investigation is still ongoing. In order not to preempt the investigation, we will not divulge the names," regional police chief Allan Nobleza told GMA News, adding that one of the suspects was linked to a local militant group.

The United States condemned what it called a "horrific terrorist attack" and said it stood with Filipinos in rejecting violence, joining a chorus of support from countries that include Japan, Australia, Britain, China and Canada.

"We mourn those killed in the attack, and our thoughts are with the injured," State Department spokesperson Matthew Miller said in statement.

This handout photo shows police inspecting the site of a bomb attack inside a gymnasium in Marawi. AFP

Daesh militants claimed responsibility for Sunday's bombing at Mindanao State University, shortly after Philippine President Ferdinand Marcos Jr said "foreign terrorists" were responsible.

Marawi is in a area known as Bangsamoro, an underdeveloped Muslim region in the predominantly Catholic Philippines, that has for decades battled with lawlessness, separatist violence and clan conflicts, prompting concern that it could become fertile ground for extremism.

Military chief Romeo Brawner on Sunday said he suspected the bombing could be a retaliatory attack as it had followed operations against local extremist groups in the southern region of Mindanao.

Reuters