A military psychiatrist urged US officers to ease harsh detention conditions for WikiLeaks suspect Bradley Manning but was overruled by brig commanders, a witness said Wednesday.

Manning, charged with passing a massive trove of secret government documents to the WikiLeaks website, is asking a judge to dismiss his case because of alleged illegal punishment he suffered during his pre-trial detention for nine months at a military brig in Quantico, Virginia.

Captain William Hoctor, a US Navy psychiatrist advising the Quantico detention center, disagreed with a decision to keep Manning under strict, maximum security conditions, saying there was no sign the accused was suicidal, a Marine officer told the court.

But Colonel Robert Oltman, one of the officers running the brig, said the doctor's view was "only one data point" and that there were other factors to take into account, including weekly reports from prison guards.

"I wasn't going to base a decision on his input alone," Oltman said under questioning by Manning's defense lawyer, David Coombs.

Oltman also said he had concerns about the doctor's credibility as Hoctor allegedly had concluded another detainee did not pose a suicide risk but the man ended up killing himself.

Manning is expected to take the stand for the first time this week during the latest round of pre-trial hearings that began Tuesday at Fort Meade, Maryland, north of the US capital.

Manning, 24, who sat in the courtroom taking copious notes during the proceedings, could spend the rest of his life in prison if convicted of aiding the enemy with the massive leak, which embarrassed the US government and rankled Washington's allies.

In the worst security breach in US history, the leaks included hundreds of thousands of military intelligence logs from Iraq and Afghanistan and roughly 25,000 sensitive diplomatic cables.

Manning's lawyer appeared to be constructing an argument at this week's hearing that officers imposed strict solitary confinement on the Army private under pressure from top brass at the Pentagon and against the advice of medical professionals and the military's own regulations.

His line of questioning honed in on the role of a three-star Marine officer at the Pentagon, Lieutenant General George Flynn, who took a keen interest in the high-profile case, according to emails cited by the defense.

Flynn made clear in emails that he wanted to be kept informed and stressed that officers must ensure that Manning did not commit suicide while detained at the Marine Corps brig, Oltman said.

When pressed by the defense lawyer, Oltman acknowledged that he would have consulted Flynn if he disagreed with any plans on Manning's detention.

The defense also questioned Oltman about an email from a legal adviser that concluded that Manning's treatment appeared to be in violation of military rules on detention.

Manning was held in a solitary cell for at least 23 hours a day and ordered to remove his underwear at night, one of a number of measures the military justified under a "prevention of injury" watch.

Oltman acknowledged an email exchange in which he and a military lawyer at Quantico joked about guards forcing Manning to strip every night, invoking a Dr Seuss verse.

The defense attorney read aloud the email sent by a military lawyer to Oltman, which offered a revised version of the children's poem:

"I can wear them in a box, I can wear them with a fox,

I can wear them in the day, I can wear them so I say,

But I can't wear them at night,

My comments gave the staff a fright."

Oltman admitted that his email reply read: "Sam I am," a reference to the main character in the Dr Seuss work.

The Marine officer said he found the reference to Dr Seuss entertaining but when asked by Coombs if Manning's treatment was a joking matter, he told the court:

"No it was not. It was a very serious issue."

Oltman said he was told by his staff that Manning had made a comment that he could kill himself with his underwear waistband or flip-flops.

But Manning's lawyer suggested his client was making the comment in a joking manner as part of a conversation in which he asked about getting his restrictive "prevention of injury" status lifted.