The earthquake, with a magnitude of 7.7, occurred on Saturday night Swedish time at a depth of ten kilometres off the island of Flores in the eastern part of the country, according to the US Geological Survey (USGS).
Buildings have collapsed, landslides have cut off major roads and several cities are without power, which has complicated rescue efforts and limited the flow of information.
"Our focus will be on finding the victims buried under the rubble," rescue leader Fathur Rahman told AFP by telephone on his way by boat to the Nagekeo area, which is the worst affected.
“Very worried”
By 11 Swedish time, 38 people had been confirmed dead, according to the country's authorities.
The earthquake has been followed by strong aftershocks in the same area, including one with a magnitude of 6.1.
Later, on Saturday afternoon Swedish time, a new earthquake with a magnitude of 6.9 occurred off the island of Sumatra in western Indonesia. No tsunami warning has been issued after the quake and it is unclear whether it caused any damage.
Alfred Wempy, who works at a hotel in the town of Maumere on the island of Flores in the province of East Nusa Tenggara, ran out into the street when the quake struck, where people were fleeing in panic to seek shelter on higher ground.
Maumere is by the sea, so we are very worried about tidal waves. I called my son and told him to take shelter. So we went to a nearby farm to save ourselves.
A small building at the hotel where Alfred Wempy works collapsed. He also says he has seen news footage of a port building in the city collapsing, and a ship tearing apart, sending passengers waiting to board falling into the water.
It was a very strong earthquake.
Affected area
Lukas Lotar works at a hospital in Maumere and says the ground suddenly started shaking.
"People were fleeing the hospital, the impact was strong. I saw some walls that had cracked," he told AFP.
Indonesian authorities have urged people to evacuate coastal areas and issued an early tsunami warning, which could later be lifted.
Indonesia, with its 17,000 islands, is located on the Pacific Ring of Fire, an area frequently hit by earthquakes and volcanic eruptions. Flores was hit by a tsunami in 1992, which killed at least 2,500 people.
