Japan Post Holdings Co. plans to acquire a 7 to 8 percent stake in U.S. insurer Aflac Inc. for about ¥300 billion as part of its strategy to expand operations overseas, sources familiar with the matter said Thursday.

Japan Post, which is in a partnership with the U.S. insurer's Japanese arm, hopes to expand its tie-up to include the joint development of insurance products by taking a stake in Aflac, the sources said.

The former state postal service hopes to complete the purchase by the end of 2019 at the earliest, according to the sources.

Under Aflac's rules, an entity having a voting right of less than 20 percent can boost its right to 20 percent after holding its stake in the U.S. company for a certain period of time. Japan Post hopes to keep its stake until it obtains a 20 percent voting right, which would make it the largest shareholder.

Under its three-year business plan through fiscal 2020, Japan Post plans to carry out business acquisitions worth hundreds of billions of yen.

As Japan Post is set to sell its holdings in Japan Post Bank Co. and Japan Post Insurance Co., it has been looking for other profit-making operations.

Japan Post's partnership with Aflac Life Insurance Japan Ltd. sees it sell Aflac's cancer insurance products at post offices.