TOKYO - Tokyo Electric Power Company Holdings (TEPCO) told Japan's nuclear regulators on Monday that it will push back the planned commercial operation of the No 6 reactor at its Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture, following signs of an electricity leak, local media reported.

The reactor marked the first TEPCO-run unit to go back online since the 2011 Fukushima nuclear disaster. It resumed operation on Jan 21 and was scheduled to begin commercial operation on Wednesday.

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But the utility decided to delay the schedule after an alarm went off last Thursday afternoon, indicating a small-scale electricity leak at a power generator, Kyodo News reported.

TEPCO President Tomoaki Kobayakawa told reporters that it remains unclear when the company will be able to begin commercial operation, noting that the investigation into the incident is underway.

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"We will stop if there are any concerns. We believe identifying the cause is the most important," he said.

The Kashiwazaki-Kariwa plant, a seven-reactor facility about 220 km northwest of Tokyo, is the world's biggest nuclear power plant by potential capacity. The restart of the No. 6 reactor in January has met local opposition amid criticism that the plant sits on an active seismic fault zone.