WASHINGTON — Northwestern Mutual Life Insurance Co. has obtained permission to operate a new, federally insured savings bank specializing in trust services.

The Office of Thrift Supervision announced Monday its approval for the company to open the bank, to be named Northwestern Mutual Trust Co. and based in Milwaukee.

The new thrift will offer trust services, which include acting as an investment adviser or custodian of trust funds or property held in trust, executing wills and acting as guardian of an estate or trustee of an IRA account.

Its main source of prospective customers will be referrals from Northwestern Mutual's 7,500 agents in all 50 states and from employees in the company's regional sales offices.

Northwestern Mutual, also based in Milwaukee, is the nation's sixth-largest life insurance company, with more than $583 billion in current insurance policies.

The federal approval makes Northwestern Mutual the latest in a string of companies in the financial services business to get new federal savings and loan charters in recent months.

Earlier this year the thrift agency, part of the Treasury Department, approved similar charters for tractor maker Deere & Co.; American Express Co.; General Motors Corp.'s consumer finance division; New York Life Insurance Co.; the Hartford Group; the National Association of Mutual Insurance Companies, a group representing insurance firms owned by policyholders; and AXA Financial Inc., parent of Equitable Life Assurance Society and brokerage firm Donaldson, Lufkin & Jenrette Inc.