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A woman who lost her house and husband in the earthquake-triggered tsunami weeps with her daughters on Pagai Island, in the Mentawai Islands, Indonesia, on Thursday. ((Tundra Laksamana/Associated Press))

Rescue crews and relief workers are losing hope of finding survivors after a tsunami swept through a remote string of islands in Indonesia, killing at least 370 people and leaving hundreds missing.

The three-metre wave roared through remote islands off Sumatra following a powerful earthquake on Monday, washing away homes and displacing thousands of people from more than 20 villages.

Joni Sageru, a 30-year-old fisherman, recalled seeing the ocean first recede and then return like "a big wall running toward our village."

"Suddenly trees, houses and all things in the village were sucked into the sea and nothing was left."

The West Sumatra provincial disaster management agency raised the official death toll to 370 Thursday, up from 343 earlier in the day. About 340 people are still missing.

Bad weather and rough seas slowed initial relief efforts, as rescue crews struggled to reach remote locations, including the Mentawai Islands, a popular surf destination.

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Tsunami survivors weep on the tsunami-ravaged Pagai Island, on Thursday. ((Achmad Ibrahim/Associated Press))

"This is a very remote area, so some areas are just getting reached," Jakarta-based freelance journalist Aubrey Belford told CBC News.

The government has deployed several cargo planes and helicopters to help deliver supplies and emergency workers, but officials are still trying to reach some of the more remote areas, Belford said.

Harmensyah, head of the West Sumatra provincial disaster management centre, said the rescue crews that had arrived in affected communities were finding bodies on the roads and beaches in devastated communities.

Harmensyah said the teams were losing hope of finding survivors. 

"They believe many, many of the bodies were swept to sea," he told The Associated Press.

On Thursday, a 10-year-old boy found an 18-month-old alive in a clump of trees — though both the toddler's parents are believed to be dead.

SurfAid International, a relief agency that works in the area, sent an assessment team to survey the damage in villages in North Pagai and South Pagai.

"Villages in the area have either been completely destroyed or suffered significant damage," SurfAid said in a statement.

Some islanders slung up tarps to sleep under in areas where the wave swept houses into the jungle. Many refused to return to their homes for fear another tsunami might hit.

Indonesian President Susilo Bambang Yudhoyono is scheduled to visit affected communities Thursday, as questions emerge about problems with the warning system put in place after the 2004 Asian tsunami.

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Officials told the BBC the multimillion-dollar warning system wasn't working properly because two buoys near the Mentawai Islands had been damaged by vandals.

However, Joern Lauterjung, head of the German-Indonesia Tsunami Early Warning Project for the Potsdam-based GeoForschungs Zentrum, said a warning did go out five minutes after the quake, but the tsunami hit so fast no one was warned in time.

"The early warning system worked very well — it can be verified," he said, adding that only one sensor of 300 had not been working, and it had no effect on the system's operation.

About 1,300 kilometres to the east in central Java, the Mount Merapi volcano was mostly quiet but still a threat after Tuesday's eruption that sent searing ash clouds into the air, killing at least 33 people and injuring 17, said Agustinus, a doctor at the local health department who, like many Indonesians, goes by one name.

Among the dead was a revered elder who had refused to leave his ceremonial post as caretaker of the mountain's spirits.

The two disasters were not related, but they both fell along Indonesia's portion of the so-called Pacific Ring of Fire, a series of fault lines that are prone to earthquakes and volcanic activity stretching from the Western Hemisphere through Japan and Southeast Asia.

With files from The Associated Press