FILE PHOTO - U.S. Treasury Secretary Steven Mnuchin speaks during a moderated discussion before the Economic Club of New York, in New York City, U.S., November 9, 2017. REUTERS/Brendan McDermid

WASHINGTON (Reuters) - U.S. Treasury Secretary Steven Mnuchin on Monday announced cash management measures to avoid a U.S. default.

In a letter to House of Representatives Speaker Paul Ryan, Mnuchin said the Treasury would no longer be able to fully invest in two retirement funds for federal workers.

They are the Civil Service Retirement and Disability Fund and the Postal Service Retiree Health Benefits Fund, according to the letter. All the funds would be made whole once the debt limit is increased, Mnuchin said.

The U.S. Treasury is bumping up against the cap on how much money it can borrow to cover the budget deficit that results from Washington spending more than it collects in taxes. Only Congress can raise that limit.

A temporary measure that suspended the debt limit expired on Dec. 8, although the Treasury has emergency means to continue to pay all its bills through January, the department has said.

The United States is one of only a few nations that requires the legislature to approve periodic increases in the legal limit on how much money the federal government can borrow.