TAIPEI ? A strong earthquake of at least 6.3-magnitude struck under the seas off Taiwan and Japan Sunday, seismologists said, but there were no immediate reports of casualties.

The Hawaii-based Pacific Tsunami Warning Center and Taiwan's seismology service did not issue any immediate tsunami warning, although the Japanese weather agency issued a minor tsunami alert.

The Japan Meteorological Agency calculated the quake's magnitude at 6.6, exceeding the US Geological Survey's estimate of 6.4 and the Taiwanese Seismology Center's estimate of 6.3.

The quake struck at 3:10 pm (0610 GMT) at a depth of 16.5 kilometers (10.3 miles) below the sea floor, 115 kilometers south of Japan's Ishigaki island and 225 kilometers southeast of northern Taiwan, the USGS said.

The quake struck in Japanese territory, south of the Okinawa island chain.

The Japanese agency said a tsunami with a height of 0.5 meters (about 19 inches) was expected along the shores of some of the archipelago's southern-most islands.

Around 20 percent of the world's most powerful earthquakes strike Japan, which, like Taiwan, lies near the junction of two tectonic plates.