TOKYO - Japan's Tokyo Electric Power Company (TEPCO) said Thursday it suspended operation at the just-restarted reactor of a nuclear power plant in Niigata prefecture after an alarm went off.

The alarm was triggered at 00:28 am local time after equipment to maneuver the control rods apparently had an issue, according to TEPCO.

TEPCO said it suspended work at reactor No 6 to withdraw control rods, which are used to adjust the nuclear fission of a reactor, and it is looking into what happened at the Kashiwazaki-Kariwa plant.

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The Nuclear Regulation Authority said the reactor is stable, adding that there are no safety problems.

No abnormal levels of radioactivity were detected around the world's biggest nuclear power station by capacity, according to the Niigata prefectural government.

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The No 6 reactor at the seven-unit complex was reactivated shortly after 7 pm local time on Wednesday, marking the first operated by TEPCO to go back online since the 2011 Fukushima nuclear disaster. The Reboot came a day later than initially planned, after a control-rod alarm also sounded during testing.