TOKYO - A strong earthquake struck off Japan's eastern coast Saturday, shaking buildings and prompting a tsunami warning, witnesses and meteorological agencies said.

There were no immediate reports of injuries or damage after the 6.6 magnitude quake, Japan's Meteorological Agency said. The U.S. Geological Survey put the strength of the initial temblor at 7.0 and said it was followed by a 5.3 aftershock. A tsunami warning was issued for parts of Japan's eastern coast. National broadcaster NHK said a small wave of 20 centimeters was reported.

The quake shook buildings several hundred miles (kilometers) away in central Tokyo, but initial reports from the scene showed no significant damage and traffic appeared to be moving normally.

Japan sits atop four tectonic plates and is one of the world's most earthquake-prone countries.

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