A survey of US soldiers in  Afghanistan has shown that troops facing three or more combat tours have higher rates of mental health and marital problems, the US army says.

The survey, released on Friday, found that 21.4 per cent of lower-ranking enlisted male soldiers, the group that generally experiences the most combat time, had mental health problems defined by army medical teams as anxiety, depression or acute stress.

That compares to 23.4 per cent in 2007 and 10.4 per cent in 2005.

And marital problems were dramatically higher among soldiers with three or more combat tours, with 30.8 per cent experiencing marital crises compared to 14.3 per cent for those on their first deployment.

In the survey soldiers said unit morale in Afghanistan had declined as the frequency of fighting had increased and combat deaths and injuries reached record highs.

'More health specialists'

Rising suicide rates and a shooting spree last week by an army psychiatrist at a base in Fort Hood, Texas, have raised new questions about the effects of combat stress and the state of the military's mental health system.

The team that carried out the surveys recommended the army deploy more mental health specialists to Afghanistan where soldiers reported increasing problems getting access to care, in order to reach a ratio of one provider for every 700 soldiers.

Currently, the ratio is one to 1,123 but officers said more than  60 mental health specialists were due to deploy, ensuring the army would meet the goal of one per 700 soldiers by the end of the year.

"We are making adjustments right now in the request for forces to come within the one to 700 ratio," Lieutenant General Eric  Schoomaker, the army surgeon general, said.

Improvement in Iraq

In contrast, the mental heath of US forces in Iraq appeared to be improving as violence declined and the military prepared for a gradual withdrawal.

The army said it saw the lowest number of psychological problems among soldiers in Iraq since 2004.

Some 13.3 per cent of junior fighters there reported anxiety, depression or acute stress in 2009, down from 18.8 per cent in 2007.