Mount Pinatubo belched steam in a weak eruption on Tuesday, a Phivolcs said as it cautioned the public from venturing into the vicinity of the volcano.
Phivolcs confirmed the eruption hours after it reported receiving a report of signals of a "weak eruption" from the Tokyo Volcanic Ash Advisory Center.
So far, no ashfall was reported from communities of Zambales.
"Local government units are advised to prohibit entry into Pinatubo Crater," 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.