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(Bloomberg) -- A spate of unusually large block trades in Japanese trading houses and insurers has put investors on edge in Tokyo.

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There have been at least six block trades in Mitsui & Co. shares totaling ¥113 billion ($765 million) since Feb. 26, and a single trade of ¥15 billion in Itochu Corp., according to data compiled by Bloomberg. Each transaction represented more than 60% of the stock’s average daily volume over 20 days.

Insurer Tokio Marine Holdings Inc. saw transactions amounting to at least ¥145 billion.

“These trades are mysterious,” said Takehiko Masuzawa, head of equity trading at Phillip Securities Japan. “We can’t pin down what’s happening.” He added that it was unusual for such large-scale block trades to happen in a row.

Block trades involve the buying and selling of a large chunk of stock at a price privately negotiated.

Mitsui’s representative said it’s not aware of the transactions, while Itochu said it does not know who was behind the block trade. A Tokio Marine spokesperson declined to comment.

Shares of the nation’s five major trading houses, including Mitsui and Itochu, jumped last week after Berkshire Hathaway Inc. said it was looking to increase ownership in the companies.

Companies, banks or insurers may be using block trades to sell their strategic share holdings, said Yusuke Sakai, a senior trader at T&D Asset Management Co.

--With assistance from Momoka Yokoyama and Nao Sano.

(Adds comment from Itochu in sixth paragraph)

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