About 30 instructors who train US sailors on the running of nuclear reactors are under investigation for cheating on a written exam, Navy commanders said Tuesday.

The allegations raised fresh questions about ethics problems in the military and come on the heels of another cheating scandal that has implicated nearly 20 percent of the Air Force's nuclear missile officers.

The suspected cheating took place at the navy's nuclear propulsion program in Charleston, South Carolina, where a sailor alerted senior officers to the problem, commanders said.

The instructors for the program, who also oversee the running of the reactors, are required to be regularly re-certified to teach fellow sailors and must pass written, oral and hands-on tests, officials said.

"The propulsion exam was allegedly shared amongst some senior enlisted operators," said Admiral John Greenert, chief of naval operations.

"To say that I'm disappointed would be an understatement," the naval chief told a news conference.

"I assure you if these allegations are substantiated we will hold the ...appropriate people accountable."

Admiral John Richardson, head of the propulsion program, declined to say at the same press briefing how many sailors were under investigation.

But a navy official, speaking on condition of anonymity, told several reporters that about 30 instructors are suspected of sharing the answers to the written exam.

Richardson said he took "full responsibility for this incident."

All sailors implicated in the alleged cheating have been "removed" from the site and stripped of access to the reactors, the admiral said.

A naval criminal investigation was under way, additional supervisors had been assigned to oversee teams at the South Carolina center and officials also were examining if the cheating reflected a broader problem.

But he said there was no doubt that the military's nuclear reactors "are operating safely."

The Navy has 10 aircraft carriers and 93 submarines powered by nuclear reactors.

Richardson said the cheating allegations were not uncovered as part of a wider Pentagon review of the nuclear force, prompted by a series of incidents in the Air Force's missile officer corps.

The cheating scandal by "missileers" in the Air Force has implicated 92 officers out of the 500-strong member corps, sparking concerns of a deeper morale problem.

But Richardson said at the moment the suspected cheating among sailors did not appear to point to any morale crisis.