The U.S. Postal Service on Friday said it posted a loss of $1.9 billion for the first three months of 2014, its 20th loss in 22 quarters, as costs continue to balloon and mail volume keeps sliding.

The service—which is mandated by Congress to prefund health benefits for future retirees—again pleaded for broad, comprehensive assistance from lawmakers. Without it, the Postal Service said, it won't be able to pay the required $5.7 billion retiree health benefit payment to the Treasury due by Sept. 30.

"We haven't been making the retiree health benefit prefunding payments because we can't," Chief Financial Officer Joseph Corbett said in a news release. "If legislation reduced the required retiree health benefit prefunding payment, it doesn't provide us with any more cash to pay down our debt or put much needed capital into our business."

Mr. Corbett said the service's $64 billion in liabilities exceed its assets by $42 billion, and said the agency needs more than $10 billion to invest in new delivery vehicles and other integral equipment.

"Some comments in recent news reports suggest that all we need from Congress is help with restructuring our retiree health benefit plan," Mr. Corbett said. "Nothing can be further from the truth."