Phivolcs said Tuesday it found signs of a "weak explosion" of Mt. Pinatubo as it cautioned the public from venturing into the vicinity of the volcano.
The incident produced a plume and happened between 12:03 p.m. and 12:13 p.m. It was reported to Phivolcs by the Tokyo Volcanic Ash Advisory Center.
Phivolcs said it is still studying the source of the explosion.
"Local government units are advised to prohibit entry into Pinatubo Crater until the source of the above explosion has been determined and to report to us any related information," Phivolcs said.
Communities surrounding the vicinity of Mount Pinatubo were advised to prepare for both earthquakes and volcanic hazards.
Mount Pinatubo is known for its 1991 eruption that was considered by the U.S. Geological Survey as the second-largest volcanic eruption of the 20th century.