The increase in the flow of lava has been accompanied by a rise in the amount of sulfur dioxide emitted by Mayon, the Philippine Institute of Volcanology and Seismology said in its latest advisory.

"One likely scenario is a shift from lava extrusion to explosive eruption," the institute warned, saying this would be accompanied by deadly mudflows of volcanic ash.

The government is still maintaining its six-kilometer (nearly four-mile) radius permanent danger zone around the volcano where no one is supposed to enter, the institute said.

However local officials and residents said that some farmers were continuing to farm the fertile slopes around Mayon despite the warning.

"Our farmers in the six-kilometer danger zone should stop for awhile and not enter there because we never know what will happen. There could be a sudden eruption," said Noel Rosal, mayor of Legaspi, the city closest to Mayon.

Rosal said in a radio interview that even wild animals such as boars and snakes were descending from Mayon and that springs on the lower slopes of the mountain were drying up, all of which were "a bad sign."

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

Alert level three of the five-step alert system has been in force around Mayon. This means that there is "relatively high unrest" in the volcano with volcanic quakes and tremors expected to become frequent ahead of an eruption.

The volcano, about 300 kilometers (176 miles) southeast of Manila, last had a major eruption in July 2001.

More than 1,000 people are believed to have perished when the volcano erupted in 1814.

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