NEW YORK (AP) -- Citigroup Inc. and the Bank of New York Mellon Corp. are trustees for unsecured creditors who are owed some $155 billion by Lehman Brothers, but both firms stressed Monday that they have little or no exposure to the failed investment bank.

Citigroup is an indenture trustee to approximately $138 million in unsecured debt owed by Lehman, the largest amount listed in a bankruptcy petition filed Monday. Bank of New York Mellon is a trustee to about $17 billion.

Seeking to clarify its role in the proceedings on a day that rattled investors, Citigroup said in a statement it was serving only as an intermediate between Lehman and buyers of its bonds and was not owed money by the bank.

"Our role in this issue is administrative in nature and does not represent exposure for Citi to Lehman," Citigroup said.

The Bank of New York Mellon also has no outstanding loans to Lehman and has "only minimal direct exposure" to the company, spokesman Ron Gruendi said in an e-mail to The Associated Press.

"Our indirect exposure primarily relates to Lehman's short-term debt held in our client portfolios," Gruendi said. "We are working to address the impact as soon as possible, and we feel that any impact will be manageable."

The issue of unsecured debt is important because unsecured bondholders are considered "impaired creditors" under U.S. bankruptcy law, meaning they won't be paid in full after a bankrupt company's assets are sold.

The remaining unsecured creditors listed in Lehman's bankruptcy petition are foreign banks, mostly from Japan.