Residents swarmed over collapsed buildings in the island's main town of Gunung Sitoli early on Wednesday, searching frantically for survivors.
About 1000 people have been killed by the quake, the regional governor said on Wednesday, adding the country would welcome foreign military help to recover from its second big disaster in just over three months.
The town's hospital was barely functioning - it lacked power or water, and fuel for generators and vehicles was running low.
"We know there are many people critically injured," said Norman Peeler, a medical coordinator from the World Health Organisation.
At a makeshift centre on a football field next to the town's palm-fringed Indian Ocean beach, 13 patients spent the night under a corrugated iron roof hoping to get on helicopter flights to a hospital on Indonesia's nearest main island of Sumatra.
But rain and cloudy weather grounded many planned flights, officials said.
"I have three critical patients with internal bleeding and serious fractures, but there is no space on the choppers," Indonesian Red Cross worker Ahmad Haris said.
The quake struck about an hour before midnight on Monday off Sumatra, some 120km north of Nias. The even-bigger quake that generated the region's devastating tsunami on 26 December hit a nearby area further northwest along the Sumatran coast.
Monday's quake initially raised fears of another tsunami and sent people scrambling for high ground in several Indian Ocean countries lashed by December's killer waves.
But no waves materialised this time.
North Sumatra governor Rizal Nurdin estimated that 1000 people died in the latest disaster, but officials feared the number could climb to 2000. Bodies were still being dug from ruins of houses and shops early on Wednesday and laid out in front of churches and mosques.
"There is no water, electricity or rice. Things are getting tough, we have had no help so what can we do?"
Marzuki Tanjung, Nias resident
Looting broke out on Nias, with men, women and children scrabbling through a two-story store and leaving with boxes of noodles, clothes and even a television set.
"There is no water, electricity or rice. Things are getting tough, we have had no help so what can we do?" said Marzuki Tanjung, who was not among the looters.
Nurdin said he had spoken to Vice-President Jusuf Kalla, who said that foreign aid, including military help, was welcome during the emergency phase. Nations including Australia and Japan have offered to send troops.
Andi Malarangeng, a spokesman for Indonesian President Susilo Bambang Yudhoyono, said the president would visit Nias on Thursday morning.
Japan is sending $140,000 worth
of aid to Nias island
Japan said on Wednesday it would send an 11-member emergency medical team and $140,000 worth of blankets, generators, sleeping pads and tents.
Australia dispatched two military transport planes with medical supplies, and diverted a transport ship to the new disaster zone.
UN agencies were trying to coordinate deliveries of food, fresh water and medical supplies by helicopter.
The agencies have stockpiles of supplies in the region to help feed and care for survivors of the 9-magnitude quake and tsunami on 26 December that killed more than 126,000 on Sumatra and left about half a million homeless.
Burying the dead
Gunung Sitoli's main mosque was turned into a mortuary for 21 Muslim victims. At a makeshift clinic outside the mosque, volunteers were running out of supplies.
"I have not slept since the earth began to shake," said Dr Lucas Sapto, an Indonesian volunteer who was treating children with cuts on their faces.
A Chinese temple had about 20 bodies laid out in the tropical heat.
"I have not slept since the earth began to shake"
Lucas Sapto, volunteer
"We are waiting for a hearse. Once it comes, I can bury my daughter and two grandchildren," said Lukmin, a 74-year-old Chinese Indonesian Buddhist.
Nias appeared to have borne the brunt of the tremor, but neighbouring islands were also hit and details of casualties there were sketchy.
Budi Atmaji Adiputro, chairman for Indonesia's Coordinating Agency for National Disaster Relief, said his office was reporting only 17 dead on Simeulue island, despite reports from a local official of 100 victims.
"We have to be careful in counting" the dead, he said, adding "we just have to count when we have seen the bodies".