Radiation levels inside the Onagawa nuclear plant in the Miyagi prefecture, north-east of Japan, rose slightly as water spilled out of spent fuel pools after a strong 7.1-magnitude aftershock hit the region on Thursday night. But the company that runs the plant said there was no rise in radiation levels outside the plant.

The epicenter of the fresh tremor was just 20 km away from the Onagawa nuclear plant, which has been safely shut down after the quake and tsunami in March.

Tohoku Electric Power, the operator of the plant, said on Friday water leaked out of spent fuel pools of three reactors at the plant after the quake.

We detected a small rise in radiation levels inside the reactor buildings, and are trying to find the locations of the leaks, a Tohoku Electric said. We see no change in radiation levels outside the reactor buildings.

It had been reported earlier that two out of three power lines to Japan's Onagawa nuclear plant were knocked out by the fresh aftershock.

The Fukushima crisis erupted after an off-site power failure led to the disruption of the cooling systems at the plant, leading to partial melt-downs and radioactive leakages.