Philippine has temporarily blocked an online gaming app that one of two students blamed for a deadly school shooting has avidly used in order to assess whether it played a role in fostering such violence.

Three students were killed and 20 others were wounded when the two suspects, aged 14 and 15 and armed with a handgun each, opened fire on Monday at the San Jose National High School in central Tacloban city.

The Cybercrime Investigation and Co-ordinating Centre said its decision to block Gorebox was prompted by an ongoing police investigation which showed that one suspect was an avid user of the app, adding that this would allow authorities to determine "whether the platform played any role in the actions of the suspects".

The decision would be enforced starting on Tuesday, undersecretary Aboy Paraiso of the cybercrime centre said in a statement.

"We cannot ignore possible online influences that may have contributed to this tragic incident," Paraiso said, without specifying how long the app would be blocked.

Paraiso did not say what action might be taken if a government assessment concluded the app promoted violence among users.

"Beyond this temporary ban, we are reinforcing our monitoring efforts to identify online spaces that may pose risks to young users and to ensure that appropriate interventions are made immediately," Paraiso said.

Regional police chief Jason Capoy said the suspects have told investigators they staged the attack to retaliate for being bullied in school.

He and other police officials, however, said a thorough investigation would look at all possibilities, including the possible influence of online groups promoting rebellion and violent behaviour among youth.

One suspect got the 9mm pistol from an aunt, a police officer, while the other had a .38 calibre revolver obtained from an employee of a security agency.

They managed to bring the guns onto the campus because it had inadequate security for the school's 1600 students, police said.

The dead and wounded were all students, police said, adding that investigators recovered at least 40 shell casings at the scene.

The suspects were to be turned over to government welfare officers after the investigation because of their age.