TOKYO (Reuters) - Japan Post Bank bought about 300 billion yen ($3.3 billion) in U.S. Treasuries in October-December, the first time it has purchased U.S. government debt since the start of its privatization process in October 2007, the Nikkei business daily said on Monday.

Japan Post Bank, a unit of Japan Post Holdings Co, typically parks roughly 80 percent of its about 190 trillion yen in assets in Japanese government bonds, and the bank bought Treasuries in order to diversify its assets, the Nikkei said.

Coupled with its insurance unit, it is the largest investor in Japanese government bonds, at 33 percent of the market and plays a pivotal role in keeping bond yields low.

Japan Post Bank apparently bought U.S. Treasuries in October-December without hedging against foreign exchange risk, the Nikkei said without citing sources.

A Japan Post spokesman said the bank now holds U.S. Treasuries but declined to comment on the amount it has bought.

Bank disclosures show it held no dollar bonds at the end of March last year and about 31 billion yen of dollar-denominated bonds at the end of September. The bank has not revealed the currency breakdown of its foreign bond holdings as of December.

The dollar traded in a range centering roughly around 90 yen in the October-December quarter and slid to a 14-year low of 84.82 yen on trading platform EBS in late November.

There has been speculation in markets that Japan Post bought U.S. Treasuries when the dollar hit its 14-year trough.

The bank's balance of "miscellaneous securities," which largely consist of foreign securities, jumped to 3.6 trillion yen as of the end of 2009, a 2 trillion yen increase from March 2009.

Even after such an increase, that category accounted for less than 2 percent of its overall assets.

Bank disclosures also show that in the past, a large amount of the foreign bonds the bank holds have been euroyen bonds, yen bonds that are issued outside Japan.

At the start of its privatization process, Japan Post Bank reduced its holdings of riskier assets such as stocks and foreign bonds, the Nikkei said. But the bank has gradually invested in overseas bonds including euro-denominated bonds and yen-denominated foreign bonds in an attempt to gain higher yields, it continued.

The bank is also believed to hold dollar-denominated corporate bonds, the newspaper added.

The Nikkei report comes at a time when the government is preparing to announce privatization plans for Japan Post, an enormous state-owned financial conglomerate that includes Japan Post Insurance as well as Japan Post Bank.

The plan, the symbol of former prime minister Junichiro Koizumi's market-friendly reforms, was put on hold after the Democratic Party-led government took power last year.

Banking Minister Shizuka Kamei has said that he wants Japan Post to diversify and that it could buy more U.S. Treasuries, but he later said its money is important for the stability of the domestic bond market.

A government report has said it is unrealistic for the behemoth to make a big change to its portfolio in a short time given its importance to the stability of the JGB market.

(Reporting by Kaori Kaneko and Hideyuki Sano; Editing by Chris Gallagher)