With negotiations over the fiscal 2011 budget collapsing, officials warned that the impact of a federal government shutdown would stretch from the Internal Revenue Service to the Smithsonian Institution to battlefields abroad.

Most federal employees still don’t know whether they would have to work during a shutdown, but Cabinet secretaries and other agency bosses began answering questions late Tuesday by distributing a Q&A regarding the effect such an event would have on federal pay and benefits.

The guidance warns that employees may not voluntarily work during a shutdown and that all essential and nonessential personnel would be paid only if Congress approved such funding after a shutdown ended. Any workers scheduled to take paid leave would not be able to, and some would be eligible for unemployment benefits if a shutdown continued for more than a few days.

A shutdown would also affect pay for members of the military, said senior government officials familiar with the planning. If the current funding expires on Friday, in the middle of the military’s two-week pay period, the Defense Department would distribute paychecks for the first week, according to the officials, who spoke on the condition of anonymity because they were not authorized to discuss the matter publicly.

In an effort to avoid such disruption, House Republicans introduced legislation that would pay troops if a deal isn’t reached. Congressional aides couldn’t say whether such a bill would pass either chamber before Friday.

Many civilian federal workers are not expected to learn their fates until Monday morning, said Colleen M. Kelley, president of the National Treasury Employees Union. If a worker is furloughed, she said, he or she would have up to four hours in most cases to collect personal belongings and leave.

Kelley learned the details during a conference call Tuesday with White House budget officials. She and other union leaders said they walked away with several unanswered questions. Most especially, union leaders wonder how departments would determine who is needed to serve on skeletal staffs during the impasse.

“It’s all over the map, and it’s not clear,” Kelley said. “If there are 50 people in one occupation and they need five as essential, how do they decide?”

Kenneth Baer, a senior official and spokesman at the Office of Management and Budget, wouldn’t elaborate on the plans, but said the administration “will continue to take necessary steps to prepare for the possibility” of a shutdown if budget negotiations fail.

Other officials familiar with plans warned privately that the IRS would cease processing refunds for paper-submitted tax returns. The Federal Housing Administration would withhold guarantees for home loans.

Managers at the National Archives said all of its operations would shutter, with the exception of workers who protect collections. Scientists at the National Institutes of Health who do clinical work would be considered essential, an agency official said.

The Department of Homeland Security would suspend its e-Verify system, which allows employers to check a worker’s immigration status, officials said. But most of DHS’s 230,000 employees hold jobs that would continue during an impasse — without pay. Agents would still protect U.S. borders, airport security guards would screen passenger bags, and the U.S. Coast Guard would continue to patrol the nation’s waters.

On Capitol Hill, lawmakers have until Friday to select which of their own staff members would work through an impasse, with all others furloughed.

A shutdown’s impact would be felt most immediately just down the street from Congress, at the Smithsonian museums on the Mall. With spring break and school trips to Washington underway, the museums are expecting 3 million visitors this month, according to Smithsonian spokeswoman Linda St. Thomas. Already 23,000 visitors have bought advance tickets for lunch and IMAX theater shows.

But if talks collapse Friday evening, St. Thomas warned that visitors probably would find museum doors locked on Saturday morning.

Staff writers Peter Finn, Lisa Rein and Felicia Sonmez contributed to this report.