The Postal Service needs help from Congress to stop the red ink, Postmaster General Patrick Donahoe told CNBC on Wednesday.
Despite an increase in package revenues and an emergency price hike that took effect in January, the agency said Monday it lost $2 billion in the second quarter.
Read More The postman bleedeth...billions in Q2
"The problem we face ... is we are required by law to prefund retiree health benefits to the tune of about $5.7 billion a year. And we don't have enough money to make that up right now. And that's the bulk of the losses," Donahoe said in a " Squawk Box " interview.
The Postal Service is down 200,000 employees over the past six years, he added. "Things that we've been able to control we've done. Now we need Congress to act to resolve the retiree health benefits."
"We are also pressing ... to go from a six day to a five day mail delivery program," he said, adding that the move would save $2 billion a year.
-By CNBC's Matthew J. Belvedere. Reuters contributed to this report.