The Philippine authorities have ordered the evacuation of about 35,000 people living near a volcano, saying an eruption could take place soon.
Mayon is the most active volcano in the Philippines
The alert was raised to four - the second highest level - following increased activity at Mount Mayon, in the centre of the country.
It has been rumbling since February and started emitting lava in mid-July.
Mayon is the most active volcano in the Philippines, having erupted about 50 times in the past 400 years.
People living in the region watched with alarm early on Monday as six successive volcanic blasts within 40 minutes sent ash up to 800m into the air.
By mid-morning, Mayon's peak was covered in a dark cloud of volcanic material rising high above the crater.
"This morning we recorded at least six small explosions and this signifies that Mayon is almost ready to burst," said Ernesto Corpus, chief of eruption prediction at the Philippine Institute of Volcanology and Seismology (Phivolcs).
Officials took the decision to raise the alert level to four, which means that an eruption could happen within days. Level five represents an ongoing eruption.
The army has sent 80 trucks to take villagers living within 8km (five miles) of the crater to 34 evacuation centres.
Officials say residents will be forced to leave
The governor of Albay province, Fernando Gonzalez, said 29 villages around the volcano would be evacuated. But provincial officials said some residents were reluctant to leave their land, fearing looters.
Defence Secretary Avelino Cruz said the evacuation was going well, but warned that police would remove those who refused to leave.
"If we have to carry them out bodily into the truck and get them out of the zone, the Task Force Mayon will do that," he told a press conference.
Officials said about 50,000 residents would be evacuated if a major eruption occurred, and a Phivolcs advisory urged people living in nearby areas to prepare to leave.
The Philippines lies on the Pacific ring of fire, where volcanic activity and earthquakes are common.
Mayon is one of 22 active volcanoes in the Philippines. Its most violent eruption, in 1814, killed more than 1,200 people. Another 75 people died during an eruption in 1993.