Defense Secretary Leon Panetta ordered US military commanders on Thursday to carry out a review of ethics training among senior officers amid a widening sex scandal that cost CIA chief David Petraeus his job.

Recent cases of misconduct involving the top brass had "the potential to erode public confidence in our leadership and in our system for the enforcement of our high ethical standards", Panetta, on a tour of Asia, wrote in a memo to the military's top-ranking officer, General Martin Dempsey.

"Worse, they can be detrimental to the execution of our mission to defend the American people," he wrote.

The announcement comes in the wake of a scandal that prompted the resignation of the former four-star general and CIA director Petraeus, as well as a litany of allegations and disciplinary action against other senior military leaders.

Panetta directed Dempsey and the chiefs of the armed services to examine "existing ethics training programmes to determine if they are adequate" and to report back with their findings within a few weeks.

The Pentagon chief then planned to issue an initial report to President Barack Obama on the issue by December 1, according to the memo released to reporters.

Recent investigations and embarrassing revelations have painted a picture of a military leadership living a privileged existence, unlike the troops they lead, with critics saying the senior officer corps has become increasingly unaccountable.

The commander of US and NATO troops in Afghanistan, General John Allen, has been caught up in the Petraeus affair, while another four-star officer, General William "Kip" Ward, was demoted to three stars this week after a probe alleged he had wasted government funds on a lavish lifestyle.

Ward, former head of US Africa Command, has been ordered to reimburse $82,000 for having inappropriately used Pentagon travel funds.

Another Army general, Jeffrey Sinclair, was removed from his post earlier this year in Afghanistan after being accused of sexual misconduct with subordinates and of threatening one woman's life.

Despite recent cases, Panetta said the vast majority of senior officers were beyond reproach.

"The majority of these officers lead by example, which is one of the reasons the United States military stands without peer," he wrote.

But Panetta said he had spoken with military leaders over the past several months "about those instances when senior officers have not lived up to the standards expected of them".

Panetta also asked Dempsey for his "views on how to better foster a culture of value-based decision-making and stewardship among senior general and flag officers and their staffs".

Panetta's trip this week to Australia, Thailand and Cambodia has been often overshadowed by the unfolding Petraeus sex scandal, with the secretary asked about the case at news conferences at each stop.

US defence officials said the initiative aimed at promoting a stronger culture of ethics had been in the works even before the Petraeus affair erupted.

"This is something that has been on his mind for some time," said a senior defence official, who spoke on condition of anonymity. "This is a first step."

In his memo to Dempsey, the chairman of the Joint Chiefs of Staff, Panetta said it was not enough for officers to comply with the ethics regulations.

"Beyond mere compliance with the rules, I also expect senior officers and civilian executives to exercise sound judgement in their stewardship of government resources and in their personal conduct," Panetta said.

"An action may be legally permissible but neither advisable nor wise."