Dai-ichi Life Co. is in advanced talks to buy U.S. insurer Protective Life Corp., in a deal likely to be worth over $5 billion, making it the biggest acquisition by a Japanese insurer yet.

Dai-ichi Life, the nation's second-largest private-sector life insurer, plans to buy 100 percent of Birmingham, Alabama-based Protective Life, a source with direct knowledge of the matter said.

If completed, the deal would be the latest in a string of overseas acquisitions by the country's insurers as they attempt to offset weak future prospects at home amid a rapidly aging population.