The US Air Force said Thursday it sacked nine nuclear missile officers and will discipline dozens of others over an exam cheating scandal that has sparked concerns about lax ethics.

The officers, including chiefs of four squadrons, were all working at Malmstrom Air Force base in Montana, which has been rocked by cheating allegations that implicated up to 100 airmen.

"Nine officers in leadership positions at Malmstrom were recommended for removal," Air Force Secretary Deborah James told a news conference.

An internal investigation had found that the commanders had not engaged in any test cheating but had "failed to provide adequate oversight of their crew force," James said.

A tenth officer had submitted his resignation over the case, she said.

A top general overseeing nuclear forces said the cheating stemmed in part from a stifling atmosphere created by commanders who over-emphasized perfect test scores for the missile launch officers.

"Although the required test score is 90 percent, crew members felt pressure to score 100 percent on each and every test," Lieutenant General Stephen Wilson said at the same news conference.

"They felt compelled to cheat to get a perfect score," he said.

The "zero-defect" approach was unrealistic and unnecessary, as the rules for managing the ballistic missiles had redundancies and other automatic safeguards, Wilson said.

"Leaders lost sight of the fact that execution in the field is more important than what happens in the class room," he said.

The cheating was first uncovered in January during an unrelated investigation into illegal drugs. The outcome of that drug probe is still pending.

Out of 100 officers potentially linked to the cheating, nine have since been cleared in an investigation, James said.

Another nine cases were being examined separately by the Air Force Office of Special Investigations. Of those, eight could involve criminal charges related to mishandling classified information -- possibly the exam materials.

About 30 to 40 officers will be retrained and allowed to return to duty to the missile force, while the remainder will face disciplinary action that could include being discharged from the military, officials said.

Pentagon chief Chuck Hagel has expressed deep disappointment over the cheating and voiced a wider concern over unethical conduct across the armed forces, after a spate of embarrassing scandals.

Hagel this week appointed a senior naval officer, Rear Admiral Margaret Klein, to serve as a special adviser looking at ethics and "character" issues.