Defense Secretary Chuck Hagel has ordered a sweeping review of the US nuclear force after a series of embarrassing incidents including cheating on an exam by missile officers, officials said Thursday.

The Pentagon chief also planned to summon top commanders as recent lapses and allegations "raise legitimate concerns about the department's stewardship of one of our most sensitive and important missions," Rear Admiral John Kirby told reporters.

"Secretary Hagel believes it is time for the Department of Defense as a whole to place renewed emphasis on examining the health of the nuclear force, in particular those issues that affect the morale, professionalism, performance, and leadership of the people who make up that force," he said.

Senior leaders overseeing the nuclear mission will meet with Hagel in the next two weeks to discuss morale and other personnel issues, he said.

After that meeting, commanders will then be required to draw up an "action plan" within 60 days to identify and remedy any personnel problems plaguing the force.

In addition, Hagel is ordering a broader "independent review" within 90 days by former officials who will look at the commanders' efforts to address professional shortcomings and any other relevant issues.

The Pentagon spokesman stressed that Hagel was focusing on morale and performance and did not believe there were safety problems related to how the Air Force manages the nuclear arsenal.

"This is, in essence, mostly a personnel-related issue that he believes we're dealing with here," Kirby said.

But it was too soon to say if some Air Force leaders needed to be held accountable over the setbacks, he said.

"Good leadership is certainly the goal. And we're just not at a point right now where we can assess specific culpability issues," he said.

Most of the incidents reported in the past several months have been associated with airmen working with the nuclear missile force but Kirby said the reviews would look at the entire mission.

The US Air Force revealed last week that 34 missile officers at a base in Montana were suspended over allegations of cheating on a proficiency exam.

The cheating was discovered as part of an unrelated criminal investigation into alleged illegal drug possession by at least 11 airmen.

In recent months, two senior nuclear commanders were sacked for misbehavior, including the head of the ICBM force after he went on a drunken bender in a trip to Russia.

And in October, officials said missile officers were caught twice failing to close the blast doors on their bunkers, violating a strict security rule.

Five years ago, the Air Force's management of the nuclear force came under scrutiny after a number of incidents showed negligent handling of atomic weapons.

The defense secretary at the time, Robert Gates, sacked the civilian secretary and the chief of staff of the Air Force over the incidents and commissioned a blue-ribbon panel to review the state of the nuclear force.

The panel concluded the nuclear mission had been marginalized since the end of the Cold War and given a lower priority, with officers often viewing the assignment as a dead-end career in the military.

The United States currently has an arsenal of 1,688 nuclear warheads, which is due to be reduced to 1,550 deployed warheads under the new Strategic Arms Reduction Treaty (START) with Russia.