The alert level was now at 2 on a scale of 5 in Mt. Kanlaon, an active volcano straddling the provinces of Negros Oriental and Negros Occidental, but residents were not advised yet to evacuate, the Philippine Institute of Volcanology and Seismology (Phivolcs) said in an advisory.
"Reports of coarse ashfall and sulfurous odors have been reported in communities on western slopes of the volcano," the agency said.
Two weak volcanic tremors were also recorded, Phivolcs added. In raising the alert level, Phivolcs said the volcano could erupt further, there could be a magmatic eruption.
The seismology agency had recommended that aviation regulators advise pilots to avoid flying close to the volcano's summit. Communities have also been advised to wear masks or cover their mouths with damp cloth to protect them from ash in the atmosphere.
Phivolcs director Teresito Bacolcol said on Monday a permanent four-kilometre danger zone was in place around the foot of the volcano, but the agency had not yet recommended evacuations for residents living near the zone.
Kanlaon, one of the country's two dozen active volcanoes, last erupted in December 2017, Bacolcol said.
The Philippines is in the Pacific "Ring of Fire," where volcanic activity and earthquakes are common.