WASHINGTON — The new spending bill passed by Congress on Thursday appears to continue the requirement for six-day mail delivery, but some lawmakers and postal officials say plans to cut Saturday service should proceed.

The financially troubled Postal Service announced last month that it would switch in August to five-day service for first-class mail and continue six-day package delivery. The government at the time was running on a temporary spending measure and postal officials invited lawmakers to spell out the way ahead in the 2013 spending bill. That sweeping funding bill was approved Thursday without new language.

Some lawmakers say a long-standing provision in the bill mandates six-day delivery. Postal authorities argue they still will have delivery over six days, just that not all mail will be delivered all six days.

Meanwhile, the Government Accountability Office weighed in with an opinion that the postal agency did not have the right to unilaterally end Saturday mail.

"We strongly disagree with the GAO's legal opinion," said David Partenheimer, spokesman for the Postal Service. "The critical issue is that the Postal Service is losing $25 million per day under its existing regulatory structure."

The letter carriers union, which has strongly disagreed with the Saturday cutback plan, sided with the GAO.

"We fully expect the Postal Service's board of governors and the postmaster general to follow the law and the expressed will of Congress about maintaining six-day delivery," Fredric Rolando, president of the National Association of Letter Carriers, said in a statement. "We do not expect to have a legal fight."

Some lawmakers believe the agency has the responsibility to make the cutback because of it mounting red ink. Among them are Sen. Tom Coburn of Oklahoma and Rep. Darrell Issa of California. The Republicans sent a letter to the postal board of governors Thursday, telling the governors to stick with their cutback plan.

"Without major, immediate restructuring actions, annual operating deficits will increase, and the Postal Service will sink much deeper into default on payments owed to taxpayers," the letter said.

The Postal Service said it expected to save $2 billion annually with the Saturday cutback. The plan accentuates one of the agency's strong points: Package delivery has increased by 14 percent since 2010, officials say, while the delivery of letters and other mail has plummeted. Email has decreased the mailing of paper letters, but online purchases have increased package shipping, forcing the Postal Service to adjust to customers' new habits.

The Postal Service lost $1.3 billion in the final three months of last year, following a nearly $16 billion loss the previous fiscal year.