By Gayatri Suroyo
JAKARTA (Reuters) - A major 7.5 quake struck off the Indonesian island of Sulawesi on Friday, briefly prompting a tsunami alert after a milder tremor brought down houses, and initial reports from the area said "many buildings" had collapsed.
The tsunami warning was lifted within the hour, but officials asked people to remain on the alert amid a series of moderate aftershocks.
"We advise people to remain in safe areas, stay away from damaged buildings," Sutopo Purwo Nugroho, spokesman for the National Disaster Mitigation Agency, said in a televised interview.
The agency was having difficulties reaching some authorities in the fishing town of Donggala and Palu city, the capital of central Sulawesi province, closest to the epicentre of the quake 80 km (50 miles) away at a shallow 10 km underground.
Palu airport was closed.
Some people took to Twitter saying they could not contact loved ones.
"My family in Palu is unreachable," Twitter user @noyvionella said.
The U.S. Geological Survey put the magnitude of the second quake at a strong 7.5, after first saying it was 7.7.
The earlier quake destroyed some houses, killing one person and injuring at least 10 in Donggala, authorities said.
More than 600,000 people live in Donggala and Palu.
"The (second) quake was felt very strongly, we expect more damage and more victims," Nugroho said, adding that evacuation process is still ongoing.
Based on initial reports, "many buildings" collapsed due to the 7.7 magnitude quake, he said.
A series of earthquakes in July and August killed nearly 500 people on the holiday island of Lombok, hundreds of kilometres southwest of Sulawesi.
Indonesia sits on the Pacific Ring of Fire and is regularly hit by earthquakes.
In 2004, an earthquake off the northern Indonesian island of Sumatra triggered a tsunami across the Indian Ocean, killing 226,000 people in 13 countries, including more than 120,000 in Indonesia.
(Reporting by Jakarta newsroom; Editing by Nick Macfie and Simon Cameron-Moore)