Defense Secretary Chuck Hagel said Friday there is “no room for error” by those responsible for America’s nuclear forces, commenting publicly for the first time on what he called “troubling lapses” in professionalism within the nuclear ranks.

Speaking at the headquarters of the military’s nuclear war-fighting command in Omaha, Neb., Hagel alluded to a series of missteps revealed in recent months, including lapses among those who operate and support the Air Force’s nuclear missile force. Last month, two senior nuclear commanders were fired amid misconduct investigations, and in August, service members working at a nuclear-missile base in Montana failed a safety and security inspection.

“You have chosen a profession where there is no room for error,” Hagel said, directing his remarks to the hundreds gathered at U.S. Strategic Command for a change-of-command ceremony. “That’s what the American people expect from you, from all of us. And you must deliver.”

Hagel did not mention specific examples of nuclear lapses, but he left no doubt that he expects the top commanders to enforce the strictest standards of behavior and performance. He said that means accepting nothing less than perfection.

“As you know, this close scrutiny and the most rigorous evaluations that we have within the Department of Defense have recently exposed some troubling lapses in maintaining this professionalism,” he said.

Hagel credited the retiring head of the command, Air Force Gen. Robert Kehler, for enforcing tough standards. Kehler began his military career in 1975 as a nuclear missile launch control officer.

Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, were at Offutt Air Force Base for the ceremony, as Kehler stepped down and Navy Adm. Cecil D. Haney took the helm.

Last month, Maj. Gen. Michael Carey was fired as commander of 20th Air Force, which is responsible for all 450 of the Air Force’s Minuteman 3 nuclear missiles. Carey was fired for behavior that officials have said is linked to alcohol abuse.

His firing and the abrupt removal just two days earlier of Strategic Command’s second-in-command — Navy Vice Adm. Tim Giardina — for alleged misconduct related to gambling came amid a series of disclosures about serious security and leadership lapses, morale problems and training flaws. In May the Associated Press disclosed an internal email in which one midlevel nuclear officer said he had found “rot” inside his Minuteman 3 missile unit at Minot Air Force Base, N.D.

Air Force and Pentagon officials insist that despite these issues, the nation’s nuclear arsenal is being operated and maintained safely.