The retirement fund of Nortel Networks Corp.’s U.S. subsidiary is being taken over by the Pension Benefit Guaranty Corp.

The PBGC said it will take responsibility for the underfunded pension plan covering 23,000 employees and retirees of Nashville, Tennessee-based Nortel Networks Inc.

The PBGC is taking action as the company, in Chapter 11 bankruptcy, prepares to liquidate. The Nortel Networks Retirement Income Plan is 58 percent funded, with assets of $716 million to cover benefit liabilities of $1.23 billion, according to PBGC estimates.