Yellen said the extended "debt issuance suspension period" would mean that Treasury would continue its suspensions of investments in the Civil Service Retirement and Disability Fund and the Postal Service Retiree Health Benefits Fund. It will also extend a suspension of the sale of State and Local Government Series (SLGS) securities.

"It is imperative that Congress act to increase or suspend the debt limit in a way that provides longer-term certainty that the government will satisfy all of its obligations," Yellen wrote in a letter to House Speaker Nancy Pelosi and other congressional leaders.

(Reporting by David Lawder; Editing by Chris Reese)