LEGAZPI, PHILLIPINES: Glowing red lava rolled down the slopes of a Philippine volcano Tuesday morning as authorities maintained a warning of a possible hazardous eruption.

The lava was quietly flowing in some places but at times Mount Mayon was erupting like a fountain, the Philippine Institute of Volcanology and Seismology said. Lava had advanced up to 2 kilometers (1.24 miles) from the crater, and ash reached up to 2 kilometers and fell on nearby communities.

Nearly 15,000 people have fled the danger zone within 6 to 7 kilometers of Mayon, and the institute strongly advised people not to re-enter the area.

Several small pyroclastic flows were generated by fragments in the lava streams and not by an explosion from the crater vent, like occurred with Mount Pinatubo, said Renato Solidum, who heads the volcano institute. Pyroclastic flows are superheated gas and volcanic debris that can race down slopes and incinerate everything in their path, and are feared in a major eruption.

"The pyroclastic flows, there were several, were not generated by an explosion from the crater with lava, molten rocks and steam, shooting up the volcano then rolling down," Solidum said. "These were generated by lava fragments breaking off from the lava flow in the upper slopes."

He also said Mayon has not seen enough volcanic earthquakes of the type that would prompt scientists to raise the alert level to four, which would indicate an explosive eruption may be imminent. Emergency response officials previously said they may have to undertake forced evacuations if the alert is raised to four.

After steam explosions Saturday and lava rising in the crater on Sunday, the alert was raised to three on a scale of five, indicating a hazardous eruption is possible "within weeks or even days."

Mayon lies in coconut-growing Albay province about 340 kilometers (210 miles) southeast of Manila. With its near-perfect cone, Mayon is popular with climbers and tourists but has erupted about 50 times in the last 500 years, sometimes violently.

In 2013, an ash eruption killed five climbers who had ventured near the summit despite warnings. Mayon's first recorded eruption was in 1616 and the most destructive in 1814 killed 1,200 people and buried the town of Cagsawa in volcanic mud.

The Philippines lies in the so-called "Ring of Fire," a line of seismic faults surrounding the Pacific Ocean where earthquakes and volcanic activity are common.

In 1991, Mount Pinatubo in the northern Philippines exploded in one of the biggest volcanic eruptions of the 20th century, killing about 800 people.