(CNN) -- Help trickled in Thursday, three days after a magnitude-7.7 earthquake struck off Indonesia, triggering a tsunami that has killed at least 311 people and left more than 400 missing.
An assessment team from the nonprofit group SurfAid International has begun surveying damaged villages on the islands of North Pagai and South Pagai, the group said Thursday.
Villages have either been destroyed or have suffered significant damage, SurfAid said.
The nonprofit is one of the few that have reached the remote disaster zone. The trip takes at least 10 hours in good conditions, according to aid agencies.
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The hard-hit Mentawai Islands region is among Indonesia's myriad islands.
"The village of Sabeugungung has been completely wiped out, with 50 dead and 150 missing presumed dead. Sikakap hospital is overflowing with over 200 people seeking assistance," SurfAid said in a blog post.
Ten villagers in Bosua had been killed, another was missing, and 78 houses had been destroyed, the nonprofit said.
Late Wednesday, SurfAid sent 1,000 shelter kits, 500 building kits and 500 hygiene kits to victims by charter boat. Another charter boat had delivered an initial batch of supplies, including blankets, tarps and water containers, and more aid shipments were planned, SurfAid said.
The group was trying to work with government officials to establish an information and coordination hub for all non-governmental organizations, spokeswoman Jossi Syahrial said.
The Indonesian Red Cross also had sent at least two aid teams to the disaster zone. One aid shipment included about 400 body bags.
Urgent needs are expected to include clean water, food, blankets, clothing, medical attention, emergency shelter and hygiene kits, said Ita Balanda, a program manager for the World Vision aid organization in Padang, Indonesia.
An estimated 7,900 households were affected, she said.
"The worst hit area is North and South Pagai Island. Reports of villages flattened are coming from there," said Antorizon of the West Sumatra disaster management agency, who like many Indonesians goes by one name.
The first pictures from North Pagai Island showed damaged and flattened structures.
Monday's quake generated a "significant" tsunami, the Pacific Tsunami Warning Center said. Some of the missing might include people who are unaccounted for after fleeing to higher ground, said Henri Dori Satoko, head of the Mentawai Islands parliament.
Though communication poses a challenge in the remote area, some witnesses in West Sumatra reported seeing a wave 6 meters (nearly 20 feet) high. Other reports described the tsunami as being about 3 meters (almost 10 feet) high.
At least one village with a population of about 200 people was swept away, with only 40 people recovered, Satoko said.
The quake struck at 9:42 p.m. Monday, triggering a tsunami warning. Its epicenter was 240 kilometers (150 miles) south of Padang, at a depth of 20.6 kilometers (12.8 miles), according to the U.S. Geological Survey. The magnitude was revised upward from a preliminary magnitude of 7.5.
"On Mentawai, a [tsunami] warning was issued seven minutes after detection of earthquake, which is what the existing system is capable of doing. Preliminary report indicates the destructive waves hit the Mentawai Islands after five minutes. ... For Mentawai villagers, the warning is late, as they are very close to the tsunami source," said Sanny Jegillos of the U.N. Development Programme. He oversees the Indian Ocean tsunami warning system, including Indonesia.
Indonesia's government has since warned residents of West Sumatra province to stay alert and stay away from the coastline for up to five days because of the possibility of more earthquakes, Balanda said.
The city of Padang and the Mentawai Islands are at the meeting place of two tectonic plates, making them vulnerable to earthquakes and tsunamis.
On December 26, 2004, a 9.1-magnitude earthquake struck off northern Sumatra. A tsunami generated by that earthquake killed more than 225,000 people in 14 countries -- mainly India, Indonesia, Maldives, Sri Lanka and Thailand. The Indonesian region of Banda Aceh was hard-hit: About 150,000 died there.
