Rescue workers were forced Thursday onto boats and helicopters to help thousands of isolated survivors of a powerful earthquake in the Philippines, as the death toll climbed to 158.

Road access to the worst-hit towns on the central island of Bohol remained cut, two days after the 7.1-magnitude quake destroyed buildings and triggered landslides that engulfed homes, regional civil defence chief Minda Morante said.

"I hope the people will understand. While we want to bring aid to them, our main adversary is accessibility," Morante told AFP.

"We acknowledge that there are still gaps in the emergency response. We cannot address the many needs all at the same time."

Morante said helicopters were being used to evacuate some of the injured as well as resupply the isolated towns with emergency food rations.

Tens of thousands of survivors had taken refuge at government-run shelters in public buildings left standing on Bohol, while others were sleeping in tents beside their homes, terrorised by aftershocks, she added.

The search for 21 missing people had narrowed down to the coastal town of Loon and neighbouring Antequera, which were close to the earthquake's epicentre, Bohol police chief Senior Superintendent Dennis Agustin told AFP.

Search and rescue teams had reached those areas by boat and narrow dirt roads over the past 24 hours, he added.

In the upland farming village of Cantam-is, about 10 kilometres (six miles) from Loon, Salvador Bonito waited on Thursday for help beside a large pile of mud, rocks and debris that buried the house of three of his friends.

"Rescuers from our church tried to reach the buried house, but we had to give up because the ground kept shifting due to aftershocks," Bonito told AFP, adding they had yet to receive outside help.

"We leave it up to God."

National Disaster Risk Reduction and Management Council spokesman Rey Balido said the number of people confirmed killed in the earthquake had risen to 158, up six from Wednesday.

Nearly all the fatalities were on Bohol, one of the country's top tourist destinations that boasts of rolling "Chocolate Hills" and tiny primates called tarsiers.

Eleven people died in Cebu, the country's second-largest city located on a nearby island of the same name, while one person was killed on the island of Siquijor, Balido said.

The quake also toppled centuries-old churches and destroyed more than 2,000 homes, the disaster council said, adding 65,000 people were staying in government-run shelters.

The Philippines lies on the so-called Ring of Fire, a vast Pacific Ocean region where many earthquakes and volcanic eruptions occur.