TOKYO, Aug 2 (Reuters) - Small tsunami waves of up to 30 cm (12 inches) hit the west coast of Japan's northernmost main island of Hokkaido on Thursday after an earthquake with a preliminary magnitude of 6.4 off Russia's Sakhalin island, the Japan Meteorological Agency said.

There were no immediate reports of damage or injury, and only light shaking was detected in some Hokkaido towns. Television cameras trained on the ocean showed no change in sea levels.

"We didn't feel the earthquake," Shinkichi Kudo, a spokesman for Wakkanai city hall in Hokkaido said by telephone. "We are warning residents of the tsunami, but we have not advised anyone to evacuate."

Waves of 30 cm were seen in Wakkanai, while smaller waves of 10 to 20 cm were also detected, NHK public television said. It warned that further waves of up to 50 cm were possible.

The earthquake took place around 11:38 a.m., the agency said.

Japan is one of the world's most seismically active nations. A magnitude 6.8 earthquake on July 16 killed 11 people in Niigata prefecture in northwestern Japan.