Updated 4:52 p.m.

Federal employee unions lashed out at the Obama administration on Monday for withholding plans for a government shutdown five 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 reveal to federal workers who would be sent home in case of a shutdown 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 enjoyed the support of government unions. As a shutdown has loomed for weeks, 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 in March 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 is skeptical.

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

So the unions are “moving forward to make sure our voices are heard,” Kelley said, urging civil servants, their families and their friends to call their senators and congress members Tuesday to express opposition to a shutdown.

OMB spokeswoman Moira Mack said last week that “it would not be prudent to discuss plans that have not been finalized.” Each agency is responsible for determining what functions are essential during a shutdown and which employees must report for duty.

Kelley said the union is collecting the private e-mail addresses of its members to reach out to them if a shutdown happens. The NTEU has also set up a Web site with information.

The American Federation of Government Employees, the largest federal union, also has put out detailed guidance to employees about what happens during a shutdown and what they should be doing to prepare.

AFGE President John Gage is scheduled to speak about the budget impasse and a possible shutdown on Tuesday at 10 a.m. at the National Press Club.