US Defense Secretary Chuck Hagel slammed Republican lawmakers as "astoundingly irresponsible" for threatening to shut down the government over a short-sighted political whim.
If Congress fails to agree on a new budget measure by the close of the fiscal year on Monday, officials estimate that about half of the Pentagon's nearly 800,000 civilian workers would be placed on unpaid leave.
The US military's nearly 1.4 million troops would stay on the job but without pay.
"When you look at the greatest democracy in the world, the largest economy in the world putting our people through this, that's not leadership, that's abdication of responsibilities," Hagel told reporters on his plane as he headed to South Korea.
"This is an astoundingly irresponsible way to govern."
Hagel, a former Republican US senator from Nebraska who enlisted to serve as a soldier in the Vietnam War, denounced those lawmakers who "want to hold the nation hostage to whatever political price they want."
He was referring to far-right tea party lawmakers who insisted they would not support a budget to fund the government if it finances President Barack Obama's sweeping health care reform.
"It is dangerously short-sighted and irresponsible, because what will this lead to in the United States of America if this continues is we will have a country that's ungovernable," Hagel said.
He expressed hope that there would be enough lawmakers who would find common ground to fund the government and avert a potentially catastrophic shutdown.
The House of Representatives was due to vote later Saturday on a Republican plan that keeps government open through mid-December while delaying implementation of Obama's health care law.
The White House has vowed to veto any such bill.
Federal agencies have made plans to furlough all non-essential employees starting Tuesday, the first day of the 2014 fiscal year, barring a vote in both chambers of Congress to fund the government beyond Monday night.