"Readiness cuts aren't always visible, but these cuts are having and will continue to have very damaging effects," he added.
Hagel made his remarks in Louisville, Ky., at the national convention of the Veterans of Foreign Wars. He said that while visiting Fort Bragg last week, he heard from infantrymen whose units didn't have enough training rounds for their weapons because of the budget cuts.
The Defense Secretary has held scores of town halls around the country in recent weeks, and at each one, sequestration has been the top concern among members of the military and their families, as well as civilian Defense Department employees.
Furloughs started on July 8 for civilians. About 90 percent of the department's workforce -- 650,000 people -- will have to take 11 days of unpaid leave before the end of September, amounting to a 20 percent pay cut.
The furloughs are expected to save the Pentagon $1.8 billion, as it works to trim $37 billion by September due to sequestration's across-the-board cuts.
"Each of the services have curtailed activities -- flying hours have been reduced, ships are not sailing, and Army training has been halted," said Hagel on Monday. "These kinds of gaps and shortages could lead to a force that is inadequately trained, ill-equipped, and unable to fulfill required missions.
Hagel is not someone who reflexively opposes Pentagon budget cuts. Long before he was nominated for Defense Secretary, he was calling for reduced spending at the Defense Department. But as he reiterated on Monday, he believes that sequestration is not the answer to the nation's budgetary problems.
"Sequestration is an irresponsible process, and it is terribly damaging. I hope that our leaders in Washington will eventually come to policy resolution, a resolution that stops sequestration," he said to applause from the audience. "But all of us who have the responsibility of leading our Defense Department cannot lead the Department of Defense based on hope, based on 'we think,' based on 'maybe.' We have to prepare our institution for whatever comes."
Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, also recently said that unless Congress addresses sequestration, there will be "a dramatic impact in our readiness."
On Tuesday, the Senate Budget Committee, chaired by Sen. Patty Murray (D-Wash.), will be holding a hearing examining the impact of sequestration and national security. One of the witnesses will be Jennifer-Cari Green, a Madigan Army Medical Center employee and single mother who is being furloughed.