KASHIWAZAKI, Japan — The strong earthquake that shook Japan's northwest coast yesterday set off a fire at the world's most powerful nuclear power plant and caused a reactor to spill radioactive water into the sea — an accident not reported to the public for hours.

Authorities today were investigating a new possible radioactive leak at the plant, the Kyodo news agency reported. A spokesman at the plant confirmed that barrels of low-level nuclear waste had tipped over but he could not give further details, such as whether there had been a leak.

The temblor killed at least nine people and injured more than 1,000 as it toppled wooden homes and tore 3-foot-wide fissures in the ground. Highways and bridges buckled, leaving officials struggling to get emergency supplies into the region.

The Japanese Meteorological Agency put the magnitude at 6.8, while the U.S. Geological Survey said it was 6.6.

Nearly 13,000 people crowded evacuation centers today amid fears of mudslides and aftershocks. Japan's weather agency issued heavy rain, flooding and lightning warnings for the area, officials said. Tens of thousands of homes lost water and power.

Chief Cabinet Secretary Yasuhisa Shiozaki said today that officials were still assessing the damage. "The most important thing is to take necessary measures quickly and respond to the needs of the victims," he said.

Kyodo, citing officials in the city of Kashiwazaki, reported that about 100 drums had fallen over at the Kashiwazaki Kariwa nuclear power plant and that the accident was discovered today, a day after the quake.

"We're currently investigating the situation and plan to deal with it as smoothly as possible," said Kensuke Takeuchi, a spokesman at the plant who refused to offer further comment.

City officials were not immediately available for comment.

The quake triggered a fire in an electrical transformer and caused a leak of radioactive water at the nuclear power plant, the world's largest in terms of electricity output.

The leak was not announced until late yesterday, about 12 hours after the quake. That fed fresh concerns about the safety of Japan's 55 nuclear reactors, which supply 30 percent of the quake-prone country's electricity and have suffered a long string of accidents and cover-ups.

About 315 gallons of slightly radioactive water apparently spilled from a tank at one of the sprawling power complex's seven reactors and entered a pipe that flushed it into the sea, said Jun Oshima, an executive at Tokyo Electric Power Co. He said it was not clear whether the tank was damaged or the water simply spilled out.

Officials said there was no "significant change" in the seawater near the plant, which is about 160 miles northwest of Tokyo. "The radioactivity is one-billionth of the legal limit," Oshima said of the leaked water.

Eliot Brenner, a spokesman for the U.S. Nuclear Regulatory Commission in Washington, said the agency told Japan's government it was ready to provide assistance if needed but had not received any request for help.

Brenner said he didn't have details about the incident. But a U.S. nuclear industry official, who spoke on condition of anonymity because the incident was a Japanese affair, said the transformer fire and water leak occurred in systems linked to different reactors.

Nine people in their 70s and 80s — six women and three men — died, most of them crushed by collapsing buildings, Kyodo said early today. One person was still missing, officials said.

Kyodo reported more than 1,000 people were hurt, with injuries including broken bones, cuts and bruises.

In Kashiwazaki, the quake reduced many older buildings to piles of lumber. Officials said today that a total of 342 houses were destroyed and another 469 damaged.

"The damage is more than we had imagined," said Kashiwazaki Mayor Hiroshi Aida. "We want to recover water first as soon as possible so more people can return home."

The area was plagued by aftershocks, but there were no immediate reports of additional damage or casualties. Near midnight, Japan's Meteorological Agency said a 6.6-magnitude quake hit off the west coast, shaking wide areas of Japan, but it was unrelated to the Niigata quake to the north and there were no immediate reports of damage.

First word of trouble at the Kashiwazaki Kariwa power plant was a fire that broke out at an electrical transformer. All the reactors were either already shut down or automatically switched off by the quake. The blaze was extinguished by early afternoon, and the power company announced there was no damage to the reactor and no release of radioactivity.

But in the evening, the company released a statement revealing the leak of radioactive water, saying it had taken all day to confirm details of the accident. The delay raised suspicions among environmentalists.

"The leak itself doesn't sound significant ... but the fact that it went unreported is a concern," said Michael Mariotte at the Nuclear Information and Resource Service, a Maryland-based networking center for environmental activists. "When a company begins by denying a problem, it makes you wonder if there's another shoe to drop."

The accident comes as the government is discussing improving the earthquake resistance of such plants, said Aileen Mioko Smith of the Japan-based environmentalist group Green Action.

The fire indicated that some facilities at nuclear power plants, such as electrical transformers, were built to lower quake-resistance levels than other equipment, like reactor cores, she said.

Economy, Trade and Industry Minister Akira Amari told the power company early today not to resume plant operations before making a thorough safety check, Kyodo reported.

The quake, which hit at 10:13 a.m., was centered off the coast of Niigata. It made buildings sway in the capital 160 miles away and was also felt in northern and central Japan. Tsunami warnings were issued, but the resulting waves were too small to cause any damage.

Some 53,000 homes in the quake zone were without water and 35,000 were without gas as of early today, local official Mitsugu Abe said.