The U.S. regulators are requesting an archive of sensitive information over the years, including how the banks calculated Tibor and their e-mails on reporting the rate, the report said on Saturday, citing several unnamed international financial sources.

The U.S. watchdogs are also investigating whether Japan's so-called "megabanks" have manipulated Libor, a benchmark that underpins transactions worth trillions of dollars, the report added.

Confidence in Libor has been shaken by revelations that bankers manipulated the rate to profit on trades and hide their own borrowing costs during the 2007-09 financial crisis.

The head of Japan's banking industry lobby said on Thursday that he does not see any particular problem with the structure of how the TIBOR is compiled.

Yasuhiro Sato, chairman of the Japanese Bankers Association and president of Mizuho Financial Group (8411.T), also said at a regular news conference he did not believe Japanese banks were involved in the manipulation of Libor.

The Japanese banking lobby has asked the 18 banks contributing to Tibor to check whether proper procedures are being followed in submitting rates.

(Reporting by Osamu Tsukimori; Editing by Ed Lane)

• A Permanent Economic Emergency