Thousands of people were evacuated from two municipalities in the Philippines on Monday as a restive volcano threatened to erupt within “weeks or even days.” The Philippine Institute of Volcanology and Seismology increased the alert level for Mount Mayon late Sunday to three on a scale of five, indicating an increased prospect of a hazardous eruption "within weeks or even days." More than 12,000 residents have already fled to evacuation centers after the 2,462-meter (8,077-foot) volcano, in central Albay province, began spewing ash on Saturday and unleashed burning mud and rocks on Sunday. “There is an ongoing forced evacuation from the municipalities of Daraga and Legazpi because of the lava flow,” Romina Marasigan, spokeswoman for the National Disaster Risk Reduction and Management Council, told reporters. “The lava flow has reached some areas around three kilometers away from the volcano.” Lava flowed at least half a kilometer (less than half a mile) down a gulley from the crater on Monday morning and ash clouds appeared mid-slope as lava fragments rolled down, said Renato Solidum, who heads the volcano institute. It was hard to track down the lava flow given the thick clouds shrouding the volcano. 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, including three Germans, who had ventured near the summit despite warnings of possible danger. Experts fear a major eruption could trigger pyroclastic flows — superheated gas and volcanic debris that race down the slopes at high speeds, incinerating or vaporizing everything in their path. More extensive explosions of ash could drift toward nearby towns and cities, including Legazpi city, the provincial capital, about nine miles (15 kilometers) away. Manila’s airport authorities said Cebu Pacific had canceled flights to nearby Legazpi City for a second day on Monday, citing bad weather. Philippine Airlines said it had not canceled any flights to and from Legazpi, but there were some delays. Mayon’s most destructive eruption was in February 1814, when lava buried a town and killed 1,200 people. It last erupted in 2014, spewing lava and forcing thousands of people to evacuate. The Philippines raised the volcano alert to level 3, which indicates increased volcanic activity toward a hazardous eruption, late on Sunday. Level 5 indicates an eruption is in progress. People have been advised to stay away from a 6-km (4-mile) radius Permanent Danger Zone and a 7-km Expanded Danger Zone on the volcano’s southern flank.