"We cannot pinpoint definitely when (it will erupt). We are in a gray area. From here on, it could either escalate or it could even subside," vulcanologist Ernesto Corpuz told Reuters.

The Philippine Institute of Volcanology and Seismology called on an estimated 18,000 people living on the volcano's slopes to "be (on the alert) against hazards from sudden explosions."

Mayon could erupt in days or weeks if its restiveness persisted, the institute said.

It declared a six-km (3.6-mile) radius from the summit a permanent no-man's land but some villagers ignored the ban, trekking back to the danger zone during daylight hours to tend their farms. They returned to their homes on the lower slopes before nightfall, relief officials said.