In this April 15, 1997 picture, Senior Airmen Mark Pacis, left, and Christopher Carver mount a refurbished nuclear warhead to the top of a Minuteman III intercontinental ballistic missile inside an underground silo in Scottsbluff, Neb. Eric Draper/AP

With public trust and safety at stake, Defense Secretary Chuck Hagel ordered immediate actions Thursday to define the depth of trouble inside the nation's nuclear force, which has been rocked by disclosures about security lapses, poor discipline, weak morale and other problems that raise questions about nuclear security.

It amounted to the most significant expression of high-level Pentagon concern about the nuclear force since 2008, when then-Defense Secretary Robert Gates fired the top uniformed and civilian officials in the Air Force following a series of mistakes that included an unauthorized flight of nuclear-armed cruise missiles across the country.

Hagel had recently said he was considering what may lay behind problems in the nuclear Air Force but his chief spokesman said Thursday that the defense secretary concluded urgent remedies were needed.

"To the degree there are systemic problems in the training and professional standards of the nuclear career field, the secretary wants them solved," the spokesman, Navy Rear Adm. John Kirby, said. "To the degree there are gaps in our understanding or implementation of those standards, he wants them closed. And to the degree leaders have failed in their duties, he wants them held to account."

Hagel summoned top military officials to a Pentagon conference, to be held within two weeks, to "raise and address" any personnel problems infesting the nuclear force, and he ordered an "action plan" be written within 60 days to explore nuclear force personnel issues, identify remedies, and put those fixes into place quickly. Hagel said he and the chairman of the Joint Chiefs of Staff, Army Gen. Martin Dempsey, will host the nuclear summit.

The Pentagon chief also said he would assemble a small group of outsiders with expertise in the nuclear field to conduct a broader review of the U.S. nuclear force, with a focus on personnel issues, and to recommend changes "that would help ensure the continued safety, security and effectiveness of our nuclear forces."

"Personnel failures within this force threaten to jeopardize the trust the American people have placed in us to keep our nuclear weapons safe and secure," Hagel wrote in a memo to a dozen top officials, including heads of the Air Force and Navy.

Since May, the AP has reported that nuclear missile force officers, from the commanding officer on down, had engaged in numerous misbehaviors or lapses. Some of those included failing security tests, violating security rules like leaving the blast door open while one of two officers napped, morale so low an officer complained of "rot" in his force and a report citing worryingly high levels of burnout.

Personnel issues are important because intercontinental ballistic missiles (ICBMs) are kept on alert every hour of every day, and the potential for human error is ever-present. Some argue that the men and women who hold the keys to the nuclear missiles have lost some of their focus on the mission, while others say their commanders are more to blame.

Not at issue, at least in the short term, is the Obama administration's commitment to keeping the bulk of the current nuclear force, which is comprised of ballistic missile submarines, nuclear-capable bombers and the Air Force's fleet of 450 Minuteman 3 ICBMs based in silos in five states.

Hagel recently reiterated his support for the nuclear force and said he is not questioning its safety.

Hagel directed his concern mainly at the Air Force and its Minuteman 3 missiles, which have been the source of many of the problems reported.

And just last week, in a disclosure that apparently startled Hagel, the Air Force said a drug investigation that began at Malmstrom Air Force Base, Mont., which operates one-third of the ICBM force, led to a separate investigation of alleged cheating on proficiency tests by 34 officers who operate the missiles there.

Those 34 officers had their security clearances suspended in a scandal that the commander at Malmstrom, Col. Robert W. Stanley II, told the AP in an interview last Friday had left his force "brokenhearted."

Last month an Air Force investigation revealed that Maj. Gen. Michael Carey, who was commander of the nuclear missile force, had engaged in embarrassing behavior while leading a U.S. government delegation to a nuclear security exercise in Russia, including heavy drinking and cavorting with suspicious women. He had been fired in October, just days after another senior nuclear officer, Navy Vice Adm. Tim Giardina, was relieved of command at U.S. Strategic Command amid allegations linked to counterfeit gambling chips.

With an eye toward avoiding further surprises, Hagel's planned Pentagon summit meeting with top officers, as well as other actions announced Thursday, include participation by Navy officials responsible for their portion of the nuclear arsenal.

The Navy has not suffered any recent reported lapses or failures within its nuclear submarine force, but Kirby said Hagel believed it would be imprudent for him not to examine the entirety of the arsenal.

"What the secretary wants to know is, what else don't I know" about problems inside the force, Kirby said.

The Associated Press