Gen Petraeus said he understood the stress troops in Iraq are under

In an open letter, Gen David Petraeus said the argument that torture could elicit information quickly was "wrong".

If the US wants to win in Iraq it must keep the "moral high ground", he said.

It comes a week after an army mental health advisory team released a survey of troops in Iraq, which found a wide tolerance for torture and abuse.

More than a third believed that torture was acceptable if it helped save the life of a fellow soldier or if it helped get information about the insurgents.

About 10% of those surveyed said they had actually mistreated Iraqi civilians by hitting or kicking them, or had damaged their property when it was not necessary to do so.

'Hasty actions'

In his one-page letter, published on a military website, Gen Petraeus said he was concerned by the results of the survey, conducted last autumn but not released until 4 May.

While we are warriors, we are also all human beings

Gen David Petraeus

He wrote: "Some may argue that we would be more effective if we sanctioned torture or other expedient methods to obtain information from the enemy. They would be wrong.

"Beyond the basic fact that such actions are illegal, history shows that they also are frequently neither useful nor necessary."

Gen Petraeus also highlighted the finding that fewer than half those surveyed would be willing to report a fellow soldier for "illegal actions", such as abusing or killing civilians.

He acknowledged the stress of battlefield situations but said troops must not hold back from reporting wrongdoing by fellow servicemen.

"Seeing a fellow trooper killed by a barbaric enemy can spark frustration, anger, and a desire for immediate revenge.

"As hard as it may be however, we must not let these emotions lead us - or our comrades in arms - to commit hasty, illegal actions."

US forces "must observe the standards and values that dictate that we treat non-combatants and detainees with dignity and respect", he added. "While we are warriors, we are also all human beings."

Mental health

High-profile allegations of mistreatment of Iraqi detainees at Abu Ghraib prison and elsewhere have damaged the US military's reputation in Iraq and the Middle East.

A hearing is currently under way in California for one of four US officers charged with dereliction of duty for not investigating the killings of 24 Iraqi civilians in Haditha in 2005.

That inquiry is just one of a number the US military has been conducting into incidents of alleged unlawful killings by US forces in Iraq.

The Pentagon report concluded that troops suffering from anxiety, depression or stress were more likely to engage in unethical behaviour.

It also found that those on extended or multiple tours were more likely to suffer mental health problems.

US Defence Secretary Robert Gates earlier this year announced that tours of duty in Iraq for active-duty troops would be extended from 12 to 15 months.