Billionaire investor Warren Buffett's Berkshire Hathaway said it added to stakes in Japan's five biggest trading houses to beyond 8% on Monday, a move likely to highlight and add to the momentum driving Japan's stock market to new heights.

Berkshire announced stakes in Itochu, Marubeni, Mitsubishi Corp, Mitsui & Co and Sumitomo in 2020 and adding in line with plans to hold the stakes long term and increase them as far as 9.9%.

Buffett's investments and his optimism about Japan's prospects have drawn attention to improving economic conditions and shareholder-friendly corporate governance reforms that have helped underpin a sparkling rally in the Nikkei share average.

The market closed 1% lower on Friday, and Berkshire's announcement came after the close, but 10 weeks of consecutive gains have helped the Nikkei rise 28% this year.

Berkshire said its ownership now averages more than 8.5% at the companies and the aggregate value of the investments is the largest of any Berkshire-held public stocks outside the U.S.

Known as "sogo shosha," Japanese trading houses trade in a variety of materials, products and food, often serving as intermediaries, and provide logistical support.

The stocks are all up more than 30% this year, with Marubeni shares up 62% and having more than tripled in price since the end of 2020. Nikkei futures slightly pared some losses after the Berkshire announcement.

(Reporting by Lavanya Ahire in Bengaluru and Tom Westbrook in Singapore; Editing by Kim Coghill and Christopher Cushing)