An earthquake with a magnitude of at least 6.2 struck off the coast of Sumatra on Saturday, triggering a tsunami alert, officials said. There were no immediate reports of a tsunami, damages or casualties.

The quake's epicenter was located off Simeulue island, about 160 miles southwest of Medan on Sumatra's northwest coast, according to the U.S. Geological Survey. It struck just after 9 p.m. local time.

Indonesia's Meteorology and Geophysics Agency put the quake's magnitude at 6.2, but the USGS said the magnitude was 6.5.

The quake strongly jolted nearby Nias island and was felt in Medan, said Subagio, an official at the Indonesian agency's Jakarta office who goes by a single name.

Simeulue island is near the epicenter of the Dec. 26 quake that caused a massive tsunami, killing or leaving missing more than 220,000 people in 11 Indian Ocean countries. Sumatra was the hardest hit, losing some 128,000 people.

The Pacific Tsunami Warning Center in Hawaii said there was no threat of a tsunami to Pacific coastlines, but quakes of this size can sometimes generate local tsunamis along coasts within a few hundred miles of the epicenter.

A team of German and Indonesian scientists are installing a tsunami early-warning system along the coast of Sumatra, and it is expected to be operational by year's end.

The system of sensors on the ocean floor and giant buoys on its surface will be able to notify coastal observation stations within 10 minutes of a tsunami-strength earthquake.

Sumatra has been wracked by scores of powerful aftershocks since last year's tsunami. Experts say the fault line that triggered that temblor is unstable and may produce another massive quake soon.