The international human rights group Amnesty International has sent a letter to US Defence Secretary Robert Gates calling for the "inhumane" conditions of US army private Bradley Manning to be reviewed.

Twenty-three year old Manning, who is accused of leaking information to WikiLeaks, has been held in a solitary cell for 23 hours a day and deprived of a pillow, sheets, and personal possessions since July 2010. He has been classified as a "maximum custody" detainee, which requires him to be shackled at the hands and legs during all visits, despite having no history of violence.

"We are concerned that the conditions inflicted on Bradley Manning are unnecessarily severe and amount to inhumane treatment by the US authorities," Susan Lee, Amnesty International’s Programme Director for the Americas, said in a media advisory.

Psychologists for Social Responsibility (PsySR) has also sent a letter to the defense secretary, asking him to "rectify the inhumane, harmful, and counterproductive treatment" of the Army private.

Solitary confinement has been used in US prisons since the 19th century, but has become more prevalent with the rise of for-profit supermax prisons in recent years. Studies have found that, depending on the prison, anywhere from 0.5 percent of US prisoners to 20 percent of prisoners are kept in 23-hour-a-day solitary confinement.

The PsySR letter noted that the UN Committee Against Torture has expressed concerns about the use of solitary confinement in US prisons, and noted that, unlike supermax prisoners, Manning has not been convicted of any crime.

He worked as a low-ranking army intelligence analyst in Iraq before being arrested in May. He was later transferred to the US Marine Corps base at Quantico, Virginia.

It was widely believed that Manning provided WikiLeaks with thousands of Iraq and Afghanistan war logs, as well as 250,000 US State Department cables. He was charged with "transferring classified data" and "delivering national defense information to an unauthorized source."

"Manning has not been convicted of any offence, but military authorities appear to be using all available means to punish him while in detention," Lee continued. "This undermines the United States’ commitment to the principle of the presumption of innocence. The repressive conditions imposed on Manning breach the US's obligations to treat detainees with humanity and dignity."

He was placed on suicide watch for two days last week against the wishes of the jail's psychiatrist, lawyer David E. Coombs told the The Washington Post.

Salon's Glenn Greenwald argued that the conditions in which Manning is held "constitute cruel and inhumane treatment and, by the standards of many nations, even torture."

"[T]he accused leaker is subjected to detention conditions likely to create long-term psychological injuries," he wrote.

Greenwald's concerns were also shared by Amnesty International and PsySR.