Army Sergeant Bowe Bergdahl was physically abused by the Taliban during his five years in captivity and is suffering deep psychological trauma, a senior US official official has told CNN.
The official, who spoke on condition of anonymity, said that at one point Bergdahl had tried to escape but was caught and confined in a small enclosure described as like a box or cage.
The claims bolster the White House's argument that president Barack Obama needed to act swiftly to ensure Bergdahl's release.
Bergdahl is currently being treated in an American military hospital in Germany where he was taken after being handed back to US troops in a prisoner swap deal on 31 May.
Doctors said that his condition is stable, but he is not yet well enough to travel back to the United States.
"There is no predetermined time line for Sgt. Bergdahl's recovery process," said a statement by the Landstuhl Regional Medical Center. "The duration will continue to be based on the pace of his healing and reintegration process."
In the US, Bergdahl's arrest has whipped up a political firestorm, with Republicans claiming he deserted his post before being captured and that the Taliban commanders released from Guantanamo Bay in the swap deal pose a risk to US troops.
In an interview, National Security Adviser Susan Rice asked the American public not to judge Bergdahl until the circumstances of his capture were known.
"This is a young man whose circumstances we are still going to learn about," she told CNN in an interview. "He is, as all Americans, innocent until proven guilty. He's now being tried in the court of public opinion after having gone through enormously traumatic five years of captivity."
She said that he will face consequences if the military finds he deserted.
"But in the meantime, let's remember this is a young man who volunteered to serve his country. He was taken as a prisoner of war. He suffered in captivity," Rice said. "He's now trying to begin the process of recovery. Let's let that happen. And then let's know the facts including his side of the story, and then we can make a judgment."
A 2009 US army investigation found that Bergdahl had deserted his post, however the conclusions are not definitive, as Bergdahl has not yet been asked about his intentions.
"And a key to that is going to be talking to Sgt. Bergdahl himself. Because there's really only one man who knows exactly what happened with respect to his disappearance and his captivity, and that's Sgt. Bergdahl himself," said Pentagon spokesman Rear Adm. John Kirby.
US officials believe that Bergdahl may have attempted to flee his captors twice.
A Taliban source told CNN that on his first attempt he had survived in the wild on his own for three days in a trench he dug with his bare hands and covered with leaves before being discovered and dragged back to captivity.
In his second attempt, he made it as far as a remote mountainous village in Pakistan before being returned to the Taliban by villagers.
The White House argues that videos of Bergdahl taken before his release show his deteriorating physical condition, and meant that the president had to act quickly and decisively to secure his release, bypassing Congress.