ISTANBUL

The Philippines’ most active volcano, Mayon, is showing signs of escalating activity, prompting authorities to reinforce exclusion zones and warn of potentially dangerous eruptions, the Philippine Institute of Volcanology and Seismology (Phivolcs) said Thursday.

Phivolcs said entry into the six-kilometer (3.7-mile) Permanent Danger Zone around the volcano is strictly prohibited, while aircraft have also been advised to avoid flying close to the summit.

Nearly 50 pyroclastic flows -- fast-moving currents of hot gas, ash and volcanic debris -- were recorded over a 24-hour period, the agency added.

Phivolcs Director Teresito Bacolcol warned of the deadly potential of such flows, recalling they killed 77 people during Mayon’s 1993 eruption, according to the Philippine News Agency.

Early Thursday, a dome-collapse event generated grayish ash plumes that rose about 1,000 meters (3,280 feet) above the crater before drifting northwest.

“Thin ashfall has been reported in Legazpi City, Ligao City, Bacacay, Guinobatan, Camalig, and the barangays of Anislag and Busay in Daraga, Albay,” Phivolcs said in a statement.

Mayon’s alert level was raised to Level 3 on Tuesday after hundreds of rockfall events were recorded in less than a week, signaling increased unrest and the possibility of hazardous eruptions.

Authorities warned of additional threats, including lava flows, ballistic fragments, landslides, explosions and fast-moving volcanic mudflows, especially during periods of heavy rainfall.

Mayon has erupted more than 50 times over the past 500 years.