"Initially the reports are that there's been a lot of destruction in terms of buildings that have collapsed as a result of this powerful earthquake," Shaista Aziz of Oxfam in London told CBS Radio News.
Oxfam, the U.N. and other relief agencies rushed to ferry aid supplies to the island, which bore the brunt of the quake's force almost three months to the day after an even bigger temblor nearby sent killer waves crashing into coastlines around the Indian Ocean's rim.
A magnitude 5.8 temblor hit off Indonesia's coast Tuesday, the latest in a series of aftershocks following the powerful earthquake that hit the region the day before, Hong Kong seismologists said.
The temblor was recorded in Hong Kong at 1:22 p.m. and was centered 217 miles south-southeast of Banda Aceh. Earlier in the day, the observatory recorded a 5.7-magnitude quake centered about 380 miles south-southeast of Banda Aceh.
The initial quake hit late at night, throwing people literally out of their beds, and many quickly took to the roads, fleeing for high ground, reports CBS News Correspondent Barry Petersen.
Fears of a second tsunami faded Tuesday when seas failed to rise up in the hours after the overnight quake — but not before triggering panic in nations still traumatized by the earlier disaster. Many took to motorbikes or motor vehicles to flee.
Buildings which were made of cement which had survived the previous earthquake collapsed and toppled, because the earthquake lasted for up to three minutes, and many people were trapped inside, reports CBS News' Stephen Fleay.
An overflight Tuesday of Gunung Sitoli, the island's biggest city, indicated about 30 percent of its buildings were destroyed, and there was significant damage in the island's second biggest town, Teluk Dalam.
Indonesia's vice president told a Jakarta radio station that the death toll could rise to between 1,000 and 2,000, based on the amount of destruction to buildings. Other officials said the dead numbered in the hundreds, not thousands.
The quake damaged Gunung Sitoli's airstrip and prevented all but small planes from landing. The Indonesian military flew The Associated Press and other news organizations over the island to inspect the damage.
Fishing villages along the coastline and inland appeared to be largely unaffected.
In Gunung Sitoli, people could be seen digging through the rubble as smoke from burning buildings hung in the air. A steeple had been knocked off a church on the mainly Christian island.
The International Organization for Migration said it was sending trucks loaded with water, milk and other food items, and medical supplies to the Sumatran port town of Sibolga, where they will be ferried to Gunung Sitoli.
Alessandra Boas, member of an Oxfam International team sent to Nias by helicopter, said the aid group was heading further afield by motorcycle.
"The devastation is obvious as soon as you land," she said. "Many of the houses here have collapsed, but it's still too early for us to get a sense of the full scale of this."
Thousands of the town's residents fled to the island's hills and remained there Tuesday.
"It's difficult to get information — all the government officials have run to the hills because they are afraid of a tsunami," presidential envoy T.B. Silalahi said.
Japan and Australia offered to send troops to Nias to help with the cleanup if Jakarta asks. India pledged $2 million in aid.
U.S. officials promised rapid assistance.
"We're applying what we've learned from the previous earthquake so that we can be prepared to be responsive quickly and in a meaningful way," deputy State Department spokesman Adam Ereli said Monday.
"We're not calling on people to give us any money at the moment," said Oxfam's Aziz. "What we're saying is that at this moment in time, we're trying to understand better what's actually happened on the ground."
The earthquake — which occurred along the same tectonic fault line as the massive 9.0-magnitude temblor that caused the Dec. 26 disaster — triggered panic in several Asian countries when governments issued warnings that another set of deadly waves may be about to hit.
Coastal residents from Indonesia to Thailand to Sri Lanka fled to higher ground when the alarm was raised, before hearing hours later that no tsunami materialized.
In many parts of Sumatra, Bande Aceh in particular, which was the main city devastated by the tsunami of three months ago, people rushed out of their homes — many were refugees already in temporary shelters — and went by motorbike and motor vehicle to higher ground, reports Fleay.
"It was horrible. The only thing on my mind was how to get out of the house immediately and save my 3½ month baby girl," said 27-year-old Marlina.
In Sri Lanka, warning sirens blared along the nation's east coast and President Chandrika Kumaratunga urged people to evacuate.
"It was like reliving the same horror of three months ago," said Fatheena Faleel, who fled her home with her three children.
By dawn Tuesday, the danger had passed and all tsunami warnings had been withdrawn.
A 5.7-magnitude reported Tuesday morning by the Hong Kong Observatory kept nervousness high.
Dave Jenkins, a New Zealand physician who runs the relief agency SurfAid International in western Sumatra, said he feared for about 10,000 people living on the tiny Banyak Islands, close to the epicenter of Monday's quake.
The Dec. 26 disaster killed at least 174,000 people in 11 countries, left more than 100,000 missing and rendered 1.5 million homeless.
Seismologists said the epicenter of Monday's earthquake was about 75 miles north of Nias. It was felt as far away as Thailand, Malaysia and Singapore.