MANILA • The Philippines has compelled residents of 12 towns and two cities near the restive Taal volcano to move to safer ground even as the intensity of the eruption has weakened.

Mandatory evacuations have been enforced in areas prone to a tsunami and a surge of gas and rock fragments if the volcano 65km south of the capital Manila erupts violently, the country's disaster management agency said.

Two people have died and more than 121,000 people have fled to safety since Sunday, but poor visibility and slippery roads due to volcanic ash are hampering evacuation efforts, the military said on Thursday.

Volcanic activity in the main crater "has been characterised by steady steam emission and infrequent weak explosions that generated dark grey ash plumes 100m to 800m tall", the Philippine Institute of Volcanology and Seismology (Phivolcs) said on Thursday.

Fissures in several villages in Batangas province have widened, a new steaming fissure was discovered on the slope of the volcano island, while the shoreline around Taal Lake has receded, the agency added.

The second-highest alert in a five-step warning system remains in effect, suggesting a hazardous explosive eruption is possible within hours or days.

"We do not base the alert level simply on what we see on the surface. We have to try to interpret what is happening below," Dr Renato Solidum, Phivolcs' chief, told CNN Philippines. "There is sometimes waning activity but the activity below is still continuing."

The nearby Cavite province has been placed under a state of emergency, joining Batangas province, which was given that status earlier this week to enable it to immediately disburse funds.

Taal is one of the most active and deadliest volcanoes in the Philippines, which lies in the Pacific Ring of Fire, where most of the world's seismic activity occurs.

The lake that once filled the crater of the volcano has almost completely disappeared owing to the interaction of water with magma that has driven a lot of the volcano's explosive behaviour since Sunday.

The disappearance of most of the lake's waters was captured in a radar image by the Finnish Iceye radar constellation, which relied on the microwave portion of the electromagnetic spectrum to peer through ejected ash and clouds at the hard surface below.

In the image, a dashed line shows the extent of the "Yellow Lake" before the onset of the eruption phase on Sunday, while a solid line outlines the extent of the lake's waters at the time the radar image was captured at 6.37 GMT on Thursday.

Other radar satellites are being used to track how the ground is deforming around the volcano. The European Union's Sentinel-1 spacecraft are able to do this by stacking repeat images, one on top of the other, the BBC reported.

This interferometric technique will help scientists understand how magma is shifting below the volcano and what that might mean for future activity.

Although Taal is one of the world's smallest active volcanoes, it can be deadly. An eruption killed more than 1,300 people in 1911.

The volcano is a tourist attraction. Hotels owned by JG Summit Holdings, Megaworld and SM Prime Holdings in the tourist city of Tagaytay near the volcano announced temporary closures in response to the warnings of a possible explosive eruption.

Businesses in several villages in Tagaytay that opt to resume operations will have to sign a waiver that says they have been briefed of the hazards, the city's disaster risk management office said.

The government will loan an initial 50 million pesos (S$1.3 million) to small businesses affected by the eruption, Trade Secretary Ramon Lopez told reporters on Thursday, adding that his agency is guarding against price spikes.

REUTERS