An earthquake of magnitude 6.6 struck an area southeast of the Indonesian city of Palu near the town of Poso on the island of Sulawesi Monday, prompting people to flee from their homes though there was no tsunami warning issued, officials said.

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Sutopo Nugroho, a spokesman for Indonesia's disaster mitigation agency, said in a statement the powerful quake lasted around 20 seconds and caused panic as people rushed out of their houses, with many staying outside because of an aftershock. Patients were evacuated from the Anutapura Hospital in Palu following the quake.

The epicenter was about 49 miles southeast of Palu in an area northwest of the town of Poso, the USGS said.

Darma Lebang, an official at Indonesia's search and rescue agency said there were no reports of casualties but according to security personnel based in Poso some older buildings in the town had been damaged.

Earlier, the Indonesian Agency for Meteorology, Climatology and Geophysics said there was no threat of a tsunami following the quake.

The U.S. Geological Survey had initially put the magnitude of the quake at 6.9, though this was later reduced to 6.6 with a depth of approximately 5.5 miles.

Indonesia is part of the so-called "Ring of Fire" in the Pacific zone prone to frequent, intense earthquakes.