(Adds tsunami threat has passed)

July 17 (Reuters) - A powerful earthquake of magnitude 7.8 off Russia's Kamchatka Peninsula triggered a tsunami warning but the threat has now passed, the U.S. Geological Survey and U.S. Pacific Tsunami Center said.

The quake struck at 11:34 a.m. on Tuesday (2334 GMT on Monday) some 125 miles (200 km) from the city of Nikolskoye on Bering island off the Kamchatka Peninsula. The epicenter was west of Attu, the westernmost and largest island in the Near Islands group of Alaska's remote Aleutian Islands.

The earthquake was very shallow, only 6 miles (10 km) below the seabed, which would have amplified its effect, but it was far from any mainland and there were no immediate reports of any casualties or damage.

The Kamchatka branch of Russia's emergency situations ministry had warned that waves up to 50 cm (1-2/2 feet) high could reach Nikolskoye.

The U.S. Pacific Tsunami Warning Center had warned earlier that "hazardous tsunami waves were possible for coasts within 300 km (186 miles) of the earthquake epicenter." But it later said that based on all available data the tsunami threat from this earthquake had passed.

The quake was initially reported as a magnitude 7.7 before being revised down to 7.4 and finally upgraded to 7.8, a major quake normally capable of causing widespread and heavy damage when striking on or near land.

The quake was followed by numerous aftershocks, including several above magnitude 5.0. (Reporting by Sandra Maler; Additional reporting by Alex Winning in Moscow; Editing by Peter Cooney and Diane Craft)