By Agence France-Presse
The official Philippine Institute of Volcanology and Seismology (Phivolcs) issued the warning as heavy rains lashed the surroundings of the Mayon volcano, which has been emitting flaming lava and giant clouds of superheated ash for about a week.
Rainwater could combine with the volcanic ash and rock to form deadly, fast-moving mudflows -- called "lahars" -- that could sweep away entire settlements, authorities said.
"If there is ash fall and heavy rain, it can be converted into (a) lahar," Phivolcs chief Renato Solidum told AFP.
"The important thing is to move out in case of heavy rains... this is a precautionary measure."
The institute earlier said in an advisory that 25 million cubic metres (about 883 million cubic feet) of ash and other volcanic material had recently been emitted by Mayon, settling on its slopes and the surrounding area.
It warned that this could result in lahars flowing into waterways and called on officials to move residents near rivers to higher ground.
An explosion of the 2,460-metre (8,070-foot) Mayon in August 2006 did not directly kill anyone but four months later, a typhoon unleashed an avalanche of volcanic mud from Mayon's slopes that claimed 1,000 lives.
Phivolcs said Mayon had emitted fountains of lava on Friday but bad weather was preventing observation of the volcano's activity on Saturday.
Volcanic mudflows have been a perennial problem during and after volcanic eruptions in the Philippines, which sits on the "Ring of Fire" -- islands in the Pacific that were formed by volcanic activity.
The government has already evacuated more than 84,000 people from a "danger zone" stretching as far as nine kilometres (six miles) around Mayon over fears of a possible deadly eruption.
Mayon, located about 330 kilometres (205 miles) southeast of the capital Manila, is the most active of the country's 22 volcanoes -- and one of the deadliest.
Four foreign tourists and their local tour guide were killed when it last erupted in May 2013.
In 1814, more than 1,200 people were killed when lava flows buried the nearby town of Cagsawa.