|PIC1|A strong earthquake killed at least three people in Japan on Monday, injured more than 500, flattened houses and started a small fire at the world's largest nuclear power plant.

Two women in their 80s died when their homes collapsed due to the magnitude 6.8 tremor, centred in Niigata prefecture some 250 km (155 miles) northwest of Tokyo, Japanese media said.

Kyodo news agency said a 79-year-old woman was also killed. Earlier reports of a fourth death were later withdrawn.

"It was too strong to stand. Some people got under tables, others immediately went outside," said police officer Masao Honma in Kashiwazaki City, near the focus of the quake.

"It's really rough," Honma told Reuters by phone.

Houses, many wooden with traditional heavy tile roofs, were flattened, a temple roof caved in and roads cracked in the quake, which was centred in the same northwestern area as a tremor three years ago that killed some 65 people.

Firemen and troops worked to rescue a woman after her voice was heard from under collapsed house, public broadcaster NHK said.

Hours after the initial quake, an aftershock with a preliminary magnitude of 5.6 jolted the area, sending more roof tiles tumbling and making it hard for some people to stand.

NHK said that more than 1,000 people had fled their homes for the nearly 100 evacuation centres that a Niigata Prefecture official said were being set up, and troops and extra emergency teams were being sent to help with rescue and relief efforts.

Buildings swayed as far away as Tokyo, and nuclear power reactors in Niigata prefecture automatically shut down for checks but there were no radiation leaks reported.

A fire in an electrical transformer at the Kashiwazaki Kariwa nuclear power plant -- the world's largest -- was quickly extinguished but it was unclear when Tokyo Electric Power Co (TEPCO) could restart three power units there, said Yoshinobu Kamijima, a company spokesman.

EMERGENCY CENTRE

Prime Minister Shinzo Abe cut short campaigning for an upcoming Upper House election and returned to Tokyo, where the government established an emergency office to deal with the quake that officials said had damaged some 350 buildings.

"We need to take every step to save lives. It's supposed to rain tomorrow in the area so we have to take every step to save lives, secure lifelines and reassure people," Abe told reporters, before heading by helicopter for the affected area.

Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes.

Bullet trains in northern Japan that had only just resumed after the initial quake were halted again by the largest aftershock. A local train toppled off the rails in the quake, but media said no one was injured.

Power and gas were cut to many homes and NHK said some 37,000 households had lost water supplies.

"We have a water tank for two days, but the city called to say they don't know when water will be running again," said Reiko Nakao, who works at a hotel in Kariwa village.

"Dishes and a TV fell off our shelves with the shaking. We haven't checked the outside wall yet, but there are cracks in the pavement around the hotel."

The 10:13 a.m. (0113 GMT) quake was centred around 60 km (37 miles) southwest of Niigata. Monday is a holiday in Japan so financial markets were closed.

Tsunami warning sirens sounded along affected stretches of the Sea of Japan but the warning was later withdrawn.

Niigata was hit in October 2004 by a quake with a matching magnitude of 6.8 that killed 65 people and injured more than 3,000. That was the deadliest quake in Japan since a magnitude 7.3 tremor hit the city of Kobe in 1995, killing more than 6,400.

Sanyo Electric Co. spokesman Akihiko Oiwa said operations had been halted at a semiconductor factory in Niigata, one of the firm's largest, but there had been no reports of damage.


(Additional reporting by Elaine Lies, George Nishiyama and Chisa Fujioka)