A strong earthquake has struck off western Indonesia, but there were no immediate reports of injuries or damage, and no tsunami warning was issued.

The U.S. Geological Survey says the quake had a magnitude of 6.4 and struck 50 kilometers (31 miles) northwest of Sikabaluan, off the coast of Sumatra island. It had a depth of 38 kilometers (24 miles).

Indonesia is prone to earthquakes due to its location on the Pacific "Ring of Fire," an arc of volcanoes and fault lines in the Pacific Basin. In December 2004, a massive magnitude-9.1 earthquake off Sumatra triggered a tsunami that killed 230,000 people in a dozen countries.