The Pension Benefit Guaranty Corp. is urging Delphi's former parent, General Motors Corp , to fulfill a commitment to absorb at least $1.5 billion in Delphi pension obligations for hourly factory workers before a September 30 deadline set by the agency.

In a letter to GM and Delphi executives, the agency said it has already asserted liens of more than $1.6 billion against the assets of Delphi's foreign subsidiaries operating outside of bankruptcy protection and was readying another $900 million in claims this week.

The letter was sent Tuesday and made available by the PBGC on Thursday.

Delphi, which filed for bankruptcy protection for its U.S. operations in October 2005, has been trying to negotiate a revised reorganization plan for the past four months after investors led by Appaloosa Management LP backed out of a $2.55 billion equity investment.

Delphi, whose debtor-in-possession financing runs through the end of 2008, faces potentially billions of dollars of claims for salaried and hourly workers' pension obligations should it not complete a transfer of liabilities to GM.

The U.S. pension agency said it would hold off on the $900 million claim if GM and Delphi filed documents with the U.S. Bankruptcy Court by Friday agreeing to a deal under which GM's pension plan would absorb at least $1.5 billion in Delphi pension liabilities.

"The parties must exert all efforts to effect the pension transfer -- the only sensible resolution that fairly protects the interests of Delphi's pensioners, the pension insurance system, and, ultimately, the other creditors and parties in interest in Delphi's bankruptcy," PBGC director Charles Millard said in the letter to GM and Delphi.

Delphi let funding waivers lapse earlier in 2008 as it was preparing to emerge from bankruptcy protection, triggering the move by the U.S. pension agency to set the September 30 deadline.

The pension agency told GM and Delphi in an August 14 letter that its claims could reach $8 billion if Delphi's hourly and salaried plans were to fail, a move that would sharply reduce what other creditors recover in the reorganization.

The PBGC insures traditional "defined benefit" pensions for about 44 million workers and retirees in over 30,000 private-sector plans. Companies pay premiums to the PBGC, which steps in when pension plans become insolvent.

When GM spun off Delphi in 1999, it agreed to assume pension and health care obligations for thousands of Delphi employees represented by the United Auto Workers union should the supplier be unable to do so.

Delphi representatives were not immediately available for comment. GM declined to comment.

(Reporting by Soyoung Kim, editing by Gerald E. McCormick)