A 6.9-magnitude earthquake struck eastern Indonesian islands on Friday, according to the US Geological Survey (USGS), but no tsunami alert was issued.

The undersea quake, at 48 kilometres deep, struck in the Molucca Sea between Sulawesi and the Maluku chain of islands.

"The quake was felt strongly for five seconds, causing people to run from their homes," the National Disaster Management Agency spokesman Sutopo Purwo Nugroho said.

Nugroho said there were no immediate reports of damage or casualties.

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