The study confirms the extent to which the US military is being strained by the wars in Iraq and Afghanistan.

The survey into the mental health of the soldiers and marines was requested by US commanders in Iraq and carried out by the office of the surgeon-general in August and October, with 1,300 soldiers and 450 marines interviewed.

The report says: "Approximately 10% of soldiers and marines report mistreating non-combatants (damaged/destroyed Iraqi property when not necessary or hit/kicked a non-combatant when not necessary).

"Soldiers that have high levels of anger, experienced high levels of combat, or screened positive for a mental health problem were nearly twice as likely to mistreat non-combatants as those who had low levels of anger or combat or screened negative for a mental health problem."

The report also found that fewer than half of all soldiers and marines would report a team member for unethical behaviour, and more than one-third believed torture should be allowed to save the life of a fellow soldier or marine.

There are about 150,000 US troops in Iraq. Many have been complaining in emails and blogs about President George Bush's decision this year to extend deployment from one year to 15 months as part of an attempt to pacify Baghdad and Anbar province.

The Pentagon this week imposed restrictions on internet postings from war zones, and claimed it was because of the risk of providing sensitive information to insurgents.

Blogs and emails from troops in the field can often be extraordinarily vivid and indiscreet.

One report last weekend from a soldier in Iraq advised a trooper in the US who was about to deploy in Iraq on ways to watch for and detect explosive devices planted by insurgents.

Reacting to the ban, soldiers said that the real reason for the curb was their negative comments about the war, including scepticism about Mr Bush's claims about progress.

Soldiers in the field and former soldiers, in blogs posted on sites such as Black Five, an unofficial site run by former paratrooper Matthew Burden, said the regulations would be inoperable, with most troops obeying the rules but dissidents finding ways round the ban.

Mr Burden, editor of The Blog of War, a book pulling together accounts from the field, criticised the decision: "No more military bloggers writing about their experiences in the combat zone. This is the best PR the military has - its most honest voice out of the war zone.

"And it's being silenced."