Federal employee unions lashed out at the Obama administration Monday for withholding plans for a government shutdown four days before Congress must agree on a budget for the rest of the fiscal year or risk furloughing workers across the country.

“Federal employees are being held hostage because these plans are not being made available,” said William R. Dougan, president of the National Federation of Federal Employees, whose members include civilian Defense Department workers. “It’s disrespectful to hold these folks in limbo until the last minute.”

He called “inexcusable” the government’s decision not to tell federal workers who would be sent home and who would be told to keep working.

It was some of the strongest rhetoric that federal unions have used against a White House that has generally had the support of government unions. As a shutdown has loomed for weeks, with the clock running out at midnight Friday, the Office of Management and Budget has ordered federal agencies to remain mum on details.

Colleen M. Kelley, president of the National Treasury Employees Union, said she was told last month by Office of Personnel Management Director John Berry and OMB Chief Performance Officer Jeffrey Zients that “plans aren’t finalized, and they don’t expect there to be a shutdown.” But she said she was skeptical.

“It seems totally incredible to me that agencies would not have their plans finalized at this point,” Kelley said.

The unions are “moving forward to make sure our voices are heard,” she said. She urged civil servants, their families and their friends to call their senators and Congress members Tuesday to express opposition to a shutdown and to a bill passed by the GOP-controlled House that calls for $61 billion in budget cuts. Employees are supposed to make the calls in their free time.

Although Obama and congressional leaders are committed to avoiding a shutdown, “given the realities of the calendar, good management requires that we continue contingency planning for an orderly shutdown should the negotiations not be completed by” Friday, Zients said.

Each agency is responsible for determining which functions are essential during a shutdown and which employees must report for duty.

During the three weeks when the government closed in 1996, 284,000 employees designated as nonessential were furloughed, union officials said. And 800,000 were told not to go to work during the five-day government shutdown in 1995. Congress had passed some spending bills before both shutdowns, which meant that many agencies were not affected. This year, though, the House and Senate have not reached agreement to fund any big agencies.

Among those sent home in 1995 were IRS employees, NTEU officials said. With two weeks until the April 18 tax filing deadline, a shutdown could cause delays in processing returns, Kelley said.

Kelley said the union was collecting the private e-mail addresses and phone numbers of its members to set up an e-mail and text-messaging system if there is a shutdown. The NTEU has also set up a Web site with information.

The American Federation of Government Employees, the largest federal union, has also issued guidance to employees about what happens during a shutdown and how they should prepare.

AFGE President John Gage is scheduled to speak Tuesday at the National Press Club about the budget impasse and a possible shutdown.