"We figure that there is now around 1000 people dead on Nias," Erni Ginting, a spokeswoman for the disaster centre for Aceh and North Sumatra, said on Tuesday.
The governor of North Sumatra province, on the main island opposite Nias, confirmed the figure and said relief efforts had been hampered by bad weather.
"The exact number cannot be confirmed, but it is around 1000 or it could be more," governor Rizal Nurdin said in provincial capital Medan.
"Visibility is only 400m. Even a helicopter finds it hard to reach," he said, adding that no flights had yet made it to the island.
Nurdin said one helicopter flight had been forced to turn back and foreign aid workers and many NGOs were hoping to fly in once the weather lifted.
More than 10,000 people fled their homes in Aceh Singkil, which lies on the other side of the epicentre from the outlying Nias island, Indonesian news agency Antara said.
Last year's tsunami left many
homeless and killed 300,000
Antara said the town centre was devastated, with private and public buildings either cracked or collapsed, electricity poles and supply down, and large cracks appearing on roads and streets, but no fatalities had been reported there yet.
Indonesian Vice-President Jusuf Kalla said between 1000 and 2000 people could have been killed in the quake - one of the eight biggest in the world since 1900.
Indonesia's westernmost province of Aceh suffered heavily in
the 26 December magnitude-9 earthquake and tsunami, which killed more than 200,000 people in the country and 100,000 more in several other Indian Ocean countries.
Monday's quake, which struck close to or after midnight across the region, spread terror in western Indonesia, Sri Lanka and coastal parts of India, Malaysia and Thailand, the areas devastated by the Boxing Day tsunami.
But unlike in the immediate aftermath of December's quake, reactions were quick in nations on the rim of the Indian Ocean, and tsunami warnings were issued across the region.
Sirens wailed and tens of thousands of panic-stricken people were evacuated after tsunami warnings, while others drove or ran from coastlines to higher ground.
But there were no signs of a tsunami up to nine hours after the 1609 GMT quake, and alerts were later withdrawn in most areas.
Indonesia's President Susilo Bambang Yudhoyono decided to delay a heavily-anticipated visit to Australia this week to go to Nias in the next one or two days, officials said.
"The visit has been postponed. The president will instead visit the disaster area tomorrow or the day after tomorrow," Susilo's spokesman Andi Mallarangeng said.
The president's visit to Australia had been seen as a breakthrough in relations between Indonesia and its southern neighbour after ties soured over Canberra's intervention during East Timor's 1999 breakaway from Jakarta.