A strong earthquake with a preliminary magnitude of 6.6 hit northwestern Japan Monday.

Japanese authorities say the quake was also felt in Tokyo and a tsunami warning was issued.

The Japanese Meteorological Agency said waves as high as 50 centimeters were believed to have hit off the coast of Niigata, but there were no immediate reports of injuries or damage

The U.S. Geological Survey says the epicenter of the quake was 220 kilometers northwest of Tokyo at a depth of 29 kilometers.

Japan is one of the world's most earthquake-prone countries.