WASHINGTON - Defense Secretary Chuck Hagel will summon the military's most senior leaders to the Pentagon to discuss serious missteps, leadership lapses and personnel problems in the U.S. nuclear force, a U.S. defense official said Thursday.

The official said Hagel also will order an independent review of the nuclear force to determine whether a string of setbacks, including numerous security lapses and other missteps revealed by The Associated Press, reflect endemic failures that could harm the nuclear mission and the safety and security of the nation's nuclear weapons.

The official disclosed Hagel's plans to the AP on condition of anonymity because he was not authorized to discuss them by name. Earlier Thursday, Hagel talked to the Air Force's top civilian official, Deborah Lee James, about setbacks in the nuclear missile force. She visited Air Force nuclear missile bases this week.

In a letter to top Pentagon leaders, including the heads of the Air Force and Navy, which together operate and manage the nation's nuclear arsenal, Hagel said he was concerned that personnel failures jeopardize Americans' trust in the military's commitment to keep the nuclear force safe and secure.

The most recent setback for the nuclear force was the Air Force's disclosure last week that 34 nuclear missile launch officers had been taken off duty because of alleged cheating on proficiency tests. That grew out of an investigation of drug use by 11 Air Force members, including three with nuclear responsibilities.