The Air Force is investigating an unprecedented exam cheating scandal involving dozens of officers responsible for launching nuclear weapons, the latest in a series of embarrassments for the military’s nuclear forces.

Air Force leaders said they stumbled upon the cheating on nuclear proficiency exams while investigating a separate drug-abuse scandal at six different bases. The disclosures come less than a month after the Air Force revealed that a two-star general in charge of nuclear missiles went on a drinking binge and fraternized with suspicious foreign women during an official visit to Moscow last summer.

Despite those problems, as well as other shortcomings involving nuclear crews in recent years, the Air Force’s top general and civilian leader sought to reassure the public Wednesday about the security and reliability of their land-based arsenal of 450 intercontinental ballistic missiles.

“The nuclear missile force remains ready and able to accomplish its mission,” Gen. Mark A. Welsh III, the Air Force chief of staff, told reporters at the Pentagon. “This is not about the compromise of nuclear weapons. It’s about compromise of the integrity of some of our airmen.”

Welsh and Air Force Secretary Deborah Lee James said 34 launch officers assigned to a nuclear-missile wing at Malmstrom Air Force Base in Montana were either caught with purloined answers to a monthly proficiency test or were aware of the cheating and did not report it. The officers — all are lieutenants or captains — have been suspended pending the outcome of the investigation.

Air Force officials said two of the officers suspected of cheating on the exam are also under scrutiny in connection with illegal drug use, along with a third nuclear launch officer at F.E. Warren Air Force Base in Wyoming. The Air Force has revealed little about the nature of the alleged drug ring, which includes other officers at other bases with no connection to nuclear arms.

Drug and cheating infractions are considered especially sensitive in the nuclear force, where launch officers have no margin for error.

Welsh and James said they learned of the cheating problems last weekend. They said they immediately ordered all 600 Air Force officers who work in missile crews to be retested on the proficiency exam by Thursday. So far, 97 percent of those who have taken the test again have passed, a normal rate, Welsh said.

“The operational capability to conduct the mission is not impacted at this point in time,” Welsh said. “The integrity issue, clearly, has got to be a concern.”

Air Force leaders said they notified Defense Secretary Chuck Hagel of the cheating scandal Wednesday morning. Hagel “was deeply troubled to learn of these allegations, and he strongly supports the aggressive steps the Air Force is taking in response to them,” Rear Adm. John Kirby, the Pentagon press secretary, said in a statement.

The 600 missile-launch officers — known as missileers — are assigned to three separate Air Force wings at Malmstrom and Warren and at Minot Air Force Base in North Dakota. The Air Force maintains about 450 Minuteman III missiles at the three bases.

Despite the reassurances from Welsh and James, the Air Force has been dogged by persistent concerns about its management of nuclear forces.

In August, the Air Force relieved a colonel in charge of a nuclear-weapons unit at Malmstrom, citing a “loss of confidence” in his leadership.

In June, a commander in charge of training missile crews at Minot Air Force Base was fired after an unusually large number of launch operators performed poorly on tests.

In 2008, Robert M. Gates, then the secretary of defense, fired the Air Force’s top general and its civilian leader after a series of nuclear gaffes occurred on their watch, including an incident in which a B-52 bomber crew flew across the country without realizing that six cruise missiles on board had been armed with nuclear warheads.

More recently, personal misconduct among officers has emerged as a problem.

In October, the Navy announced that it had removed Vice Adm. Timothy M. Giardina from his post as deputy commander of the U.S. Strategic Command, which oversees all nuclear-armed missiles, bombers and submarines, including the Air Force’s arsenal.

Giardina was placed under investigation by the Naval Criminal Investigative Service after a casino in Iowa allegedly caught him using $1,500 in counterfeit gambling chips. The casino is near the Strategic Command headquarters in Omaha.

Another senior nuclear commander, Air Force Maj. Gen. Michael Carey, was also relieved in October as the chief of the 20th Air Force, which is based at Warren and is responsible for maintaining and operating intercontinental ballistic missiles.

At the time, the Air Force said Carey was under scrutiny for “personal misbehavior” but divulged few details. Two months later, however, the service released an investigative report that found Carey repeatedly drank too much, insulted his hosts and committed a string of other gaffes during a three-day official visit to Moscow in July.