TOKYO - Japanese regulators will seek help from their US counterparts in investigating suspected insider trading by American investors in Tokyo Electric Power Co. (TEPCO) stock, a report said Saturday.

Japan's Securities and Exchange Surveillance Commission suspects that non-public information on TEPCO's 2010 issuance of roughly 500 billion yen (S$8.0 billion) in new shares leaked from Nomura Securities, the lead underwriter for the offering, the business daily Nikkei said.

The information was given to the US investors through a Japanese consulting firm, the daily added.

The investors, whose names were not mentioned, were involved in short-selling and other transactions in TEPCO stock, the daily added.

They could face tens of millions of yen in fines if found to have engaged in insider trading.

If the US Securities and Exchange Commission confirms the illegal practice, the Japanese watchdog will make what will be its first-ever call for fines against a foreign investor, Nikkei said.

TEPCO, the operator of the crippled Fukushima Daiichi nuclear power plant, is due to be effectively nationalised in July with an injection of one trillion yen in public money.

The Japanese regulatory body is also reportedly probing US investment bank JPMorgan for its alleged role in a high-profile insider trading case.

The 2010 case is among several at the centre of a wide-ranging investigation by Japan's securities watchdog into insider trading.

The commission recommended on Tuesday that an asset manager be fined for short-selling Nippon Sheet Glass shares after illegally obtaining information ahead of a stock sale JPMorgan was underwriting.

A sales executive for the US bank - which is already reeling from a shock US$2.0 billion loss on derivatives trading - was the source of the leak, Dow Jones Newswires has reported, citing an unnamed source.