The U.S. Postal Service, facing mounting losses, said Thursday that it is moving ahead with plans to close hundreds of mail-sorting hubs and cannot wait for Congress to pass legislation to help it out of its financial hole.

But the closing of 232 processing centers will be phased in over three years instead of at the faster pace postal officials had announced in the fall. About 13,000 jobs will be lost from the closures, mostly through attrition, officials said.

“After input from our customers, we’ve modified our approach,” Postmaster General Patrick R. Donahoe said at a news conference. “But the sobering reality is that the first-class-mail [volume] will not return.

“We simply do not have the mail volumes to justify the size and capacity of our current mail-processing network.”

The agency will consolidate — with full-scale closures in some cases and smaller operations in others — 140 mail-processing centers in the next year, starting with 48 in July, officials said. The rest will occur in January and February, after the fall election and the busy holiday mailing season.

Eighty-nine hubs will close or shrink operations in 2014.

Postal officials released a list of the first round of closures late Thursday.

Postal unions and some members of Congress repeated their calls that the closures represent piecemeal efforts to address the Postal Service’s financial problems. The agency is on track to lose $14 billion this year, much of it from a requirement that it pre-fund health benefits for future retirees.

“Dismantling the network and reducing services to Americans and their businesses is not a business plan,” Fredric Rolando, president of the National Association of Letter Carriers, said in a statement. He noted that almost all of the $3.2 billion loss the agency reported for the first quarter of 2012 are outlays required by the retiree mandate.

“The USPS and congressional response ought to address the actual problem,” he said.

The consolidations will affect delivery of first-class mail to some degree, officials said, with some letters arriving in three days instead of two. Local delivery would not change.

The modified approach to the mail-hub plan follows an announcement last week that the Postal Service is backing off plans to close 3,700 mostly rural post offices and instead will slash hours at 13,000 post offices. The agency had planned to close 252 mail-processing centers beginning this summer, but at the request of members of Congress put a moratorium on the closings until this week to give lawmakers time to pass legislation.

But Congress is stalled over a bill. The Postal Service does not need congressional approval to close plants and post offices, but with many lawmakers concerned about the loss of jobs in their districts, the agency agreed to hold off until this week.

(Go to to see the list of closings.)