MANILA - A "hazardous eruption" of Mount Mayon is possible within weeks or days, scientists said Monday, as thousands were evacuated along the path of the country's most active volcano.

A bright glow from the crater indicated that lava had started to flow after 3 steam-driven eruption blew ash plumes into the air over the weekend, Phivolcs director Renato Solidum told ANC.

Alert level 3 was raised last Sunday, meaning "there is always the tendency for a hazardous eruption," Solidum said. The highest warning, level 5, means an eruption is ongoing.

A hazardous eruption is "possible within weeks or even days," according to a separate Phivolcs statement.

"What we would like to see is if there would enough pressure that might propel and explode the magma coming out of the volcano," Solidum said.

• Mayon belches more ash as residents flee

Lava flows had heralded Mayon's hazardous eruptions in 1984 and 2001, which sent tens of thousands to evacuation centers, Solidum noted.

Some 938 families or 3,338 individuals have been evacuated to nearby shelters, Claudio Yucot, director of the Office of Civil Defense-Bicol Region, told DZMM.

A 7-kilometer extended danger zone has been recommended on the volcano's southern flank. The public is urged to be vigilant and avoid entering the area "due to the danger of rockfalls, landslides and sudden explosions or dome collapse that may generate hazardous volcanic flows", Phivolcs said.

Civil aviation authorities were told to advise pilots against flying near the summit due to the danger of erupting ash.

The 8,070-foot Mayon, located about 330 kilometers southeast of Manila, has a long history of deadly eruptions.

Four foreign tourists and their local guide were killed when Mayon erupted in May 2013.

In 1814, more than 1,200 people were killed when lava flows buried the town of Cagsawa.

An explosion in August 2006 did not cause direct deaths, but 4 months later a typhoon unleashed an avalanche of volcanic mud from Mayon's slopes that killed 1,000 people.

With a report from Agence France-Presse