It said water sloshed out of spent fuel pools in the plant's No.1, No.2 and No.3 reactors, which had been shut down after the 9.0 magnitude quake on March 11, and had also leaked in three other locations in the No.3 reactor complex.
The Onagawa plant, in Miyagi prefecture, was automatically shut and later cooled down safely after it was hit by a tsunami 13 meters (43 ft) higher than its base level triggered by the March 11 quake.
The operator had reported similar leakage of water from spent fuel pools and at other facilities at the plant after the March 11 quake. But engineers removed leaked water and cleaned up the contaminated area by March 31.
Tohoku Electric said on Friday two out of three lines supplying off-site power to the Onagawa site -- in so-called cold shutdown since the March 11 quake -- were lost last night in the strongest aftershock so far of the earlier quake.
Cooling operations of its spent pool fuels resumed after they stopped due to the quake, it said, and there was still an emergency backup generator to fall back on.
"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 official said. "We see no change in radiation levels outside the reactor buildings."
The International Atomic Energy Agency (IAEA), citing information from Japanese authorities, said off-site power was lost at some other nuclear facilities in the country after Thursday evening's quake, and that emergency power supply was operating.
"The epicenter of the earthquake was 20 km (12 miles) from the Onagawa nuclear power plant and approximately 120 km (75 miles) from the Fukushima Daiichi and Daini nuclear power plants," it said.
At the Fukushima Daiichi plant, which operator Tokyo Electric Power Co is struggling to stabilize after the March 11 quake and tsunami, the IAEA said it was confirmed that no changes had been observed at on-site radiation monitoring posts after Thursday's aftershock.
(Reporting by Mayumi Negishi, Yoko Nishikawa and Risa Maeda; Editing by Michael Watson)