Berkshire Hathaway has increased its stakes in Japan’s biggest trading houses, a move that is expected to offer support to the wider Tokyo stock market.

Shares of major trading houses Mitsubishi, Marubeni, Mitsui, Itochu and Sumitomo climbed 4% Tuesday morning in Tokyo, after filings to the Finance Ministry showed Monday that Berkshire’s average holding across the stocks had increased by just over percentage point to about 9.3%.

Investors had been speculating over Warren Buffett’s next move since his annual letter to shareholders said that Berkshire was looking to increase ownership in Japan’s five largest trading houses "over time.”