Major budget cuts will endanger the US military's ability to conduct its missions, Pentagon chief Chuck Hagel warned.
His comments came hours before President Barack Obama authorized cuts in domestic and defense spending, following the failure of efforts to clinch a deal with Republicans on cutting the deficit.
Hagel, whose budget at the Pentagon is set to be slashed by roughly $46 billion, said earlier: "Let me make it clear that this uncertainty puts at risk our ability to effectively fulfill all of our missions."
In contrast with his predecessor Leon Panetta, who branded the cuts a "doomsday mechanism" and "fiscal castration," Hagel was more measured two days after taking office as defense secretary.
But he made clear his thoughts on the consequences of the so-called "sequester" on the military.
Defense officials say they will be forced to reduce the working week of 800,000 civilian employees, scale back flight hours of warplanes and postpone some equipment maintenance.
The deployment of a second aircraft carrier to the Gulf has also been canceled.
The US Navy will gradually stand down several hundred planes starting in April, the Air Force will curtail flying hours and the Army will cut back training for all units except those deploying to Afghanistan.
"This will have a major impact on training and readiness," Hagel said. "Later this month, we intend to issue preliminary notifications to thousands of civilian employees who will be furloughed."
Hagel acknowledged that the budget cuts "will cause pain, particularly among our civilian workforce and their families."
"I'm also concerned, as we all are, about the impact on readiness that these cuts will have across our force," he added.
The Pentagon chief expressed "confidence" that the White House and President Barack Obama's Republican foes in Congress would eventually reach agreement.
But other officials laid bare the consequences.
"If you stop training for a while and you're a combat pilot, then you lose your rating and eventually can't fly at all, because we can't allow you to fly if you can't fly safely," said Ash Carter, Hagel's deputy at the Pentagon.
"You can't, you obviously can't fly proficiently, but you can't even fly safely. Then you have to go back to the long building-back process of getting your readiness back."
Obama was bound by law to initiate the automatic, indiscriminate cuts.
The hit to military and domestic spending, known as the sequester, was never supposed to happen, but was rather a device seen as so punishing that rival lawmakers would be forced to find a better compromise to cut the deficit.
But despite a looming reality that the nation would suffer, Democrats and Republicans remained far from compromise and were never close to agreement.