An earthquake with a magnitude of 6.5 shook buildings and caused momentary panic in the Indonesian port city of Padang on Thursday, officials and residents said, but there were no immediate reports of damage or injury.

The Pacific Tsunami Warning Center did not issue an alert immediately after the quake and Indonesia's BMK weather agency said there was no threat of a tsunami.

The quake was centered about 155 km (95 miles) south of Padang, off the coast of Sumatra island at a depth of about 50 km (30 miles), the USGS said. It had originally been reported with a magnitude of 6.2.

Memories are still fresh in Indonesia of the massive 9.15 magnitude undersea quake that triggered an Indian Ocean tsunami which killed more than 200,000 people in a dozen countries.

Most of those killed were in the province of Aceh on Sumatra's northwest tip.

A Reuters witness reported initial panic after the latest quake, which struck before dawn and lasted about 30 seconds. Residents rushed out of their homes and into the streets, but with no apparent signs of damage of injury, things quickly returned to normal.

Indonesia straddles the so-called "Pacific Ring of Fire", a highly seismically active zone, where different plates on the earth's crust meet and create a large number of earthquakes and volcanoes.