The death toll from a tsunami that pummelled remote Indonesian islands is expected to pass 500, said an official on Thursday as questions mounted over whether a warning system had failed.

Hopes were fading for hundreds of people still listed as missing after a huge wave triggered by a powerful earthquake hit the remote Mentawai islands on Monday off the west coast of Sumatra.

Meanwhile, on the island of Java in the centre of the disaster-prone archipelago, a volcano which this week killed 32 people again spewed ash and deadly heat clouds, but there were no reports of damage.

Disaster response officials said bodies were being found on beaches and coastal areas in the Mentawais, where the tsunami had washed away entire villages.

“Ten minutes after the quake we heard a sound just like an explosion from outside — it was then we realised there was a tsunami,” said 20-year-old housewife Chandra on North Pagai, one of the two worst-hit islands.

Dazed and exhausted, she was searching for her six-month-old baby boy, who has not been seen since the disaster. Neighbours found her husband's body in their village of Muntei Baru Baru.

“I know he's dead but I keep praying he's still alive. I'm so tired. I've not eaten for two days,” she told AFP.

The official death count rose to 370 with 338 missing, but disaster management official Ade Edward said the toll would climb possibly by as much as 200.

“Of those missing people we think two-thirds of them are probably dead, either swept out to sea or buried in the sand,” he said.

“When we flew over the area yesterday [Wednesday] we saw many bodies. Heads and legs were sticking out of the sand, some of them were in the trees. If we add another 200 to the toll it would be at least 543 dead.”

Survivors said they had almost no warning that the three-metre wall of water was bearing down on them, despite the laying of a sophisticated network of alarm buoys off the Sumatran coast.

The expensive system of tsunami warning buoys was established after the 2004 Asian tsunami, which killed at least 168,000 people in Indonesia alone.

An official tsunami warning was issued after Monday's 7.7-magnitude quake but it either came too late or did not reach the communities in most danger.

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