The Tokyo Electric Power Co has confirmed that water containing radioactive material leaked from a reactor at a nuclear power plant after a strong earthquake struck northwest Japan.
TEPCO had earlier said there were no radiation leaks at the plant, where all reactors were shut down after a fire prompted by the tremor.
The company said in a statement that 1.5 litres of contaminated water had been released into the ocean with 'no effect on the environment'.
Today is Ocean Day in Japan and many offices and schools were closed for the public holiday when the earthquake struck.
Seven people, all elderly, were killed when 6.8 magnitude quake struck off the coast of Niigata, which is situated about 250km northwest of Tokyo.
Nearly 300 buildings were flattened by the earthquake, which also triggered mudslides. Up to 900 people were injured.
Services on Japan's famed Shinkansen train lines were temporarily suspended as a precaution after the earthquake, which severed power to some 25,000 households. One local train derailed but there were no reports of injuries.
Amid dozens of aftershocks, thousands of people are spending the night in schools and other temporary shelters.
The quake triggered a small 50cm tsunami and was followed by 65 minor aftershocks, and a strong 5.6 tremor later in the afternoon.
Meanwhile, a strong earthquake measuring 6.6 on the Richter scale has been recorded off the Japanese coast, in the East Sea.
The Japanese meteorological agency said the tremor occured 370km under the seabed, adding that there was no risk of a tsunami.
There were no immediate reports of casualties or damage .
Japanese PM witnesses damage
Following the initial quake, Japanese Prime Minister Shinzo Abe broke off from election campaigning to visit the scene of the worst damage in Niigata and Nagano prefectures.
Mr Abe, who is struggling in opinion polls, gave instructions to his government that 'all possible measures be taken to ensure the safety of residents'.
Japan lies at the junction of four tectonic plates and is hit by about 20% of the world's most powerful earthquakes.
Niigata was hit in October 2004 by another 6.8 magnitude earthquake that killed 65 people and injured more than 3,000.
That was the deadliest quake in Japan since a magnitude 7.3 tremor hit Kobe city in 1995, killing more than 6,400.
