An earthquake of magnitude 6 hit southwestern Japan on Thursday, bringing down some buildings, killing at least two people and injuring hundreds, local media said, but the nuclear regulator reported no problems at power plants.

The initial tremor struck 11 km (7 miles) east of the city of Kumamoto, according to the U.S. Geological Survey (USGS). It said the magnitude was 6.2 but later revised it down.

There was no tsunami warning, but one person was killed after being crushed by a collapsing building, and the other by a fire that broke out after the quake, with at least 400 people being treated at local hospitals, public broadcaster NHK said.

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NHK showed footage of firefighters tackling a blaze in a building in Mashiki, a town of about 34,000 people near the epicenter of the quake.

"We will do our utmost and carry on with life-saving and rescue operations throughout the night," Prime Minister Shinzo Abe told reporters.

Japan ese media showed residents, some of them wrapped in blankets, huddling in parking lots and other open space for fear of further building collapses.

"The apartment building I live in is now tilting. Everything fell down inside. It's a mess," a male resident in Mashiki said on NHK.

About 16,500 households in and around Mashiki were without electricity as of 2 a.m. (1700 GMT), according to Kyushu Electric Power Co Inc.

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The Nuclear Regulation Authority said there were no irregularities at three nuclear plants on the southern major island of Kyushu and nearby Shikoku.

In March 2011, a quake of magnitude 9 struck offshore north of Tokyo, causing tsunami waves along the coast that killed nearly 20,000 people and triggered a nuclear power plant meltdown.

After Thursday's quake, some high-speed trains were halted as a precaution.

Honda Motor Co suspended output at its motorcycle factory near Kumamoto following the quake, a company spokesman said. Honda planned to inspect production equipment at the factory, which has an annual output capacity of 250,000 units, on Friday to see how soon production can be resumed.

Mitsubishi Electric Corp and tire maker Bridgestone Corp also suspended operations at their factories in the area, Kyodo news agency said.