Standard & Poor's, a private credit rating agency, assigned an AAA financial strength rating to Tokio Marine Life Insurance Co. Thursday, giving it the highest rating for a domestic life insurance firm.

Alico Japan, a member of the U.S.-based AIG Group, also has an AAA rating. One major reason behind the rating was the support that could be expected from its parent firm, the nation's top nonlife insurer, Tokio Marine and Fire Insurance Co., S&P said.

Tokio Marine itself has been assigned the agency's AAA rating, and the firm's outlook is stable, the rating company said.