Nippon Life Insurance Co. aims to strike a deal this month to buy National Australia Bank's insurance business by next March, sources close to the matter said.

The major Japanese life insurer, which last month agreed to buy smaller rival Mitsui Life Insurance Co. by March for ¥300 billion to ¥350 billion, is expected to spend ¥200 billion to ¥300 billion ($2.5 billion) for the Australian insurer, the sources said Tuesday.

Under a three-year management plan, Nippon Life is poised to spend up to ¥1.5 trillion on mergers and acquisitions through March 2018, and will also consider buying a U.S. life insurer.

National Australia Bank is one of that country's major banks and offers financial services to wealthy and corporate clients. It posted a group net profit of 5.3 billion Australian dollars ($3.8 billion) for the business year to September 2014.

It is looking to sell its insurance business amid international moves to tighten control on the sector, while Nippon Life wants to expand overseas to shore up revenue under threat from Japan's rapidly aging and declining population.

Nippon Life lost its position as industry leader in Japan in terms of life insurance premiums to rival Dai-ichi Life Insurance Co. in the past business year to March 31 for the first time in the postwar era.

Dai-ichi Life in February bought midranking U.S. life insurer Protective Life Corp. for $5.55 billion in the largest-ever acquisition by a Japanese life insurer of a foreign company.