A 6.4-magnitude earthquake struck off the west coast of Indonesia's Sumatra island on Wednesday, sending terrified residents fleeing from homes and buildings.

One man collapsed and died of shock while rushing out of his home, while several schools and other buildings suffered various degrees of damage, authorities said.

The quake, at a depth of 22 kilometres (14 miles), struck around 07:30 am (0030 GMT), some 34 kilometres northwest of Sinabang, capital of Simeulue -- a small, remote island off Sumatra's west coast, the US Geological Survey (USGS) said.

Residents of Sinabang said violent shaking was felt for about a minute and had caused an electricity blackout.

"People were crying, grabbing their belongings and rushing out of their homes," said an AFP reporter in Sinabang.

"There is no electrical power in some areas and I can see cracks in the walls of homes around me," he said.

"Some people are returning inside their homes, but others are still outside, afraid there may be aftershocks."

National Disaster Agency spokesman Sutopo Purwo Nugroho said one man died while fleeing from his home on the island, which has a population of about 80,000.

"A man on Simeulue died of shock. He had a history of high blood pressure and was rushing out of his home when he collapsed and died," Nugroho said.

He said the worst damage occurred on western coastal districts of Simeulue, where "a junior high school is severely damaged, with partially collapsed walls, severe cracks and a collapsed ceiling".

Nugroho added that "minor to severe damage" was spread over three districts of the island, where several schools, health facilities, mosques and other buildings suffered damaged.

He said there were no other immediate reports of casualties directly related to the quake.

Indonesia's meteorology agency said the quake occurred in the sea off Simeulue, but did not issue a tsunami warning. It said slight shaking was felt in the city of Banda Aceh, about 360 kilometres from Simeulue.

The Australian Bureau of Meteorology said the Indonesian quake posed no tsunami threat to Australia.

The USGS initially reported a 6.6-magnitude quake, but downgraded the intensity to 6.4 -- the same strength Indonesia's meteorology agency reported.

Indonesia sits on the Pacific "Ring of Fire" where continental plates collide, causing frequent seismic and volcanic activity.