With time running out for congressional leaders to reach a budget deal, the country's biggest federal workers union is taking the Obama administration to court for what it called a failure to explain how federal programs and services would be affected if there's a government shutdown.

The American Federation of Government Employees announced Tuesday that it filed a lawsuit in federal court last week seeking details on the shutdown plans of federal agencies.

The union's national president, John Gage, said he filed the lawsuit after the White House Office of Management and Budget failed to respond to a Freedom of Information Act request sent early last month.

Gage said public unions have been given very little information on how agencies would determine which employees are essential in the event of a government shutdown.

"It's not something that should be cavalierly handled," Gage said in a written statement. "If a shutdown goes on, there will be federal employees who are going to be hurt financially. They should know before the eve of a shutdown what is happening and it should be done orderly and not in a last-minute rush."

The Office of Management and Budget declined to comment on the lawsuit, saying it doesn't talk about pending litigation. But the office said the administration "has taken unprecedented steps to improve government transparency and accountability, and shutdown planning is no exception. We still believe that there is an opportunity to avoid a costly government shutdown, which would cause undue harm to the lives of federal employees, the services millions of Americans rely on, and the economic recovery under way."

The agency called plans for a shutdown "pre-decisional at this time," adding that more information will be released as it is finalized.

With Republicans and Democrats unable to reach a budget agreement, a government shutdown could occur as early as this weekend. A top White House budget official has written a memo to agency heads directing them to review and share their contingency plans for a shutdown.

Gage said he sent a letter to Attorney General Eric Holder last week, stating the union's position that requiring federal employees to work without getting paid violates the 13th Amendment, which prohibits involuntary servitude. He said he also sent a letter last week to Office of Personnel Management Director John Berry asking how long "essential employees" can be required to work during shutdown without being paid.

Gage said if there is a shutdown, the union will be calling on employees to report to their job sites early next week to demonstrate that they are ready to go back to work.

"The American public can see at least that federal employees know the importance of their jobs and that political games should not be played with their services or with their jobs," he said.