The agent said several soldiers under Colonel Shields' command at Joint Base Lewis-McChord had admitted to illegal use of steroids. One of the suspected users was a battalion captain.

Colonel Shields, who led the 4th Battalion, 23rd Infantry Regiment, was sceptical. He questioned whether investigators might have mistaken legal dietary supplements for steroids.

But in the days that followed, the captain, as well as a lieutenant, first sergeant and nine other soldiers, admitted using steroids, according to documents that offer a rare look at illegal use of those drugs in the military.

Four Australian special forces soldiers and three other soldiers have also been sent home in disgrace from Afghanistan since June after being caught abusing steroids. All special forces troops were tested for steroid use after the first four were caught in June.

Steroid use in the US Army has been on the rise. To prepare for - and perform - on combat tours of duty, some soldiers told investigators they turned to steroids to boost their brawn.

The latest US Defence Department survey - conducted in 2008 - found that 2.5 per cent of army personnel had illegally used steroids within the past 12 months, a jump from three years earlier, when 1.5 per cent said they had used these drugs.

Several soldiers from the 4/23 Battalion who confessed to using steroids estimated that more than half the 700-soldier unit had sampled steroids, according to investigative documents obtained under the US Freedom-of-Information Act.

Anabolic steroids can increase muscle mass and strength but they are typically taken at much higher levels than those prescribed by doctors. These drugs can raise the risk of high blood pressure, heart and liver disease, and side effects can include mood swings, irritability and increased aggression, which can be a volatile attribute for soldiers headed off to battle.

Since 2008 only about 300 soldiers have been tested for steroids. The army conducts random testing of 450,000 soldiers each year for use of marijuana, cocaine, heroin and other narcotics. Army officials say the steroid analysis is too expensive to be included in the random drug testing. An army spokeswoman said the captain, first lieutenant and first sergeant who used steroids underwent disciplinary actions and did not go to Afghanistan.

McClatchy Newspapers WAR PROGRESS

A Defence Department report on Afghanistan described progress in the war as ''uneven'' and painted a more sobering picture than comments made recently by the President, Barack Obama, and US military officials. The report spoke of ''modest gains in security, governance and development in operational priority areas'', but outlined challenges such as corruption and Afghan army recruitment.