A continuous stream of lava from Sunday night until early Monday and increased seismic activity indicated "heightened unrest of the volcano which could lead to explosive eruption," the Philippine Institute of Volcanology and Seismology warned.

The lava flow was accompanied by 314 tremors, the institute said.

"This was significantly higher than the 111 tremor episodes recorded yesterday," it said.

The institute also noted a high sulfur dioxide reading and observed that the volcano was extruding lava relatively quickly.

The chief of the National Disaster Coordinating Council, Glen Rabonza, said government agencies and the military were prepared to carry out mass evacuations if Mayon erupts and that contingency plans were in place. "We are ready to implement those contingency plans," Rabonza told reporters.

He reiterated a call for residents to stay away from a six-kilometer (nearly four mile) danger zone around the volcano and stand ready for any evacuation.

The 2,462-meter (8,077-foot) high volcano, famous for its near-perfect cone shape, began spewing lava on Friday.

An alert level of three in a five-step alert system is in force around Mayon. The alert level means the volcano is priming for a major eruption.

Mayon, about 300 kilometers (176 miles) southeast of Manila, last had a major eruption in July 2001, while more than 1,000 people are believed to have perished when the volcano erupted in 1814.

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