Radioactive water from the fuel pools of two reactors at the Kashiwazaki-Kariwa Nuclear Power Plant in Japan's Niigata prefecture spilled over after a magnitude 7.6 earthquake and multiple aftershocks rocked the country on New Year's Day.

However, Tokyo Electric Power Company, which owns and operates the plant, said the reactors were offline before the quake hit and no abnormalities in operation had been detected.

The plant on Japan's northern coast is TEPCO's only workable nuclear power plant since the March 2011 earthquake and tsunami destroyed its Fukushima Daiichi plant and caused its Fukushima Daini plant to cease operations.

Monday's powerful temblor and series of aftershocks killed at least 48 people and damaged thousands of buildings, with rescue teams struggling on Tuesday to reach isolated areas where buildings had been toppled, roads wrecked and power cut to tens of thousands of homes, Reuters reported.

In Beijing, Foreign Ministry spokesman Wang Wenbin said at a regular news conference that China extends its condolences to Japan over the loss of lives and "sympathies to the families who have lost loved ones and to those who have been injured".

He said the Chinese embassy in Tokyo and the consulates in Nagoya and Niigata immediately activated the emergency response mechanism and released consular notices.

"So far, there has been no reports of Chinese casualties," he added.

The quake with a preliminary magnitude of 7.6 struck on Monday afternoon, prompting people in coastal areas to flee to higher ground as tsunami waves hit Japan's western seaboard, sweeping cars and houses into the water.

A 3,000-strong rescue team comprising army personnel, firefighters and police officers has been sent to the quake site on the Noto peninsula in Ishikawa prefecture, according to Reuters.

The Japan Meteorological Agency said it has detected around 200 tremors between Monday and Tuesday, and warned of more aftershocks in the coming days.

The Kashiwazaki-Kariwa plant is one of the world's largest nuclear plants in terms of output. It is about 400 kilometers from Noto.

TEPCO said that about 10 liters of water containing radioactive materials spilled from the fuel pool of the No 2 reactor and about 4 liters from the fuel pool of the No 7 reactor at around 6:45 pm on Monday.

After the 2011 earthquake triggered a triple meltdown at the Fukushima Daiichi plant, Japan introduced a maximum operating life of 60 years for its nuclear reactors. However, the limit was scrapped in May as the nation shifted its policy to promote nuclear energy, aiming for an early restart of nuclear power plants, The Yomiuri Shimbun reported.

Mo Jingxi in Beijing contributed to this story.