Bradley Manning, the US soldier suspected of being behind the largest leak of state secrets in history, has entered his ninth month in military detention and continues to be held in maximum security conditions that critics claim are in violation of his human rights.

Manning spends 23 hours of every day in his windowless 6.7 square metre cell, which contains nothing but a bed and blanket, sink and toilet. He is allowed no personal objects other than one book or magazine at a time and is prevented from taking any exercise other than in the one hour a day allocated to it, when he is taken to an empty room and allowed to walk around it in a figure of eight.

He also remains on what is known as "prevention of injury" or POI watch which means guards check him every five minutes and wake him at night if he is not fully visible. For two days last month, against the advice of prison psychiatrists, he was placed on full suicide watch, which involved him being stripped to his underwear and having his glasses confiscated unless reading or watching television.

On the rare occasions he has visitors, he has to be shackled by hand and foot and be accompanied by two guards at all times.

Manning was arrested in Iraq where he was working as an intelligence analyst at the Operating Base Hammer. He is alleged to have been the source of several WikiLeaks releases, including the massive trove of US diplomatic cables last November.

So far, however, he has only been charged with illegally obtaining more than 150,000 cables and transferring them to an unnamed "unauthorised person".

Since his arrest on 29 May last year, lawyers and campaign groups have protested against his treatment at the hands of his military jailers, initially in Kuwait and then, from July, at the brig at the Marine Corps base in Quantico, Virginia. Amnesty International has called on the British government to intervene in his treatment on the grounds that his Welsh mother makes him a UK citizen. It called his regime "unnecessarily harsh and punitive", pointing out that he has no record of suicidal or violent behaviour in custody.

Manning's lawyer, David Coombs, has lodged a complaint that depicts his treatment as abuse and demands that his status is downgraded from maximum security to medium custody.

One of the few people to have been allowed to visit Manning in Quantico, David House, has witnessed the soldier's deterioration over the past few months. He told the Guardian recently: "Each time I go there seems to have been a remarkable decline. That's physical too. When I first saw him he was bright-eyed and strong like he was in early photographs, but now he looks weak, he has huge bags under his eyes and his muscles have turned to fat. It's hard watching someone over the months sicken like that."

In his most recent visit, House tweeted that Manning was in a "shocked state" as a result of his confinement, "but his mood and mind soared when I mentioned the democratic uprisings in Egypt".

Further detail of the visit was given by the blogger Jane Hamsher, who reported that according to House the prisoner was starting to show signs of prolonged isolation. He was slow to respond and seemed emotionally withdrawn.

Manning's lawyer is now hoping that a change of leadership at the top of the brig section of the marine base will bring a rethink of his treatment, and a shift to a more lenient regime. His military trial is not anticipated until May at the earliest.