WASHINGTON, Feb 25 (Reuters) - A magnitude 6.9 aftershock hit off Indonesia's Sumatra island on Monday, the latest in a series of powerful quakes to hit the area, the U.S. Geological Survey reported.

It said the quake, quite shallow at a depth of only 19.9 miles (32 km), was centered 107 miles (172 km) southwest of Padang on Sumatra.

There was no immediate tsunami warning.

At least two strong earthquakes struck the same area earlier on Monday, triggering panic and a brief tsunami warning, although there were no immediate reports of damage or casualties.

A huge earthquake measuring more than 9 struck off Sumatra on Dec. 26, 2004, causing a massive tsunami and more than 230,000 deaths in countries across the region.

(Reporting by Sandra Maler; Editing by Frances Kerry)

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