WASHINGTON (AFP) - Secretary of State John Kerry defended on Sunday the prisoner swap that freed US soldier Bowe Bergdahl, said to have been sometimes kept in a metal cage and in total darkness while in captivity.

Mr Kerry doubled down on President Barack Obama's controversial decision to release five Taleban detainees held at Guantanamo Bay a week ago in exchange for the freedom of Bergdahl, who the top US diplomat said was at risk of being tortured by his captors.

"It would have been offensive and incomprehensible to consciously leave an American behind, no matter what, to leave an American behind in the hands of people who would torture him, cut off his head, do any number of things," Mr Kerry told CNN's State Of The Union programme.

Mr Kerry made the remarks - his first on the controversial prisoner exchange - amid a news report about the grim conditions in which Sgt Bergdahl was held while a hostage in Afghanistan.

The New York Times wrote that the army sergeant told medical officials he was kept in total darkness in a metal cage for weeks, as punishment for trying to escape.

Sgt Bergdahl, in treatment at an army facility in Landstuhl, Germany, is healing physically, but is still emotionally too fragile to be reunited with his relatives, said the daily, citing anonymous US officials who have been briefed on his condition.

The Wall Street Journal said Bergdahl has so far declined opportunities to speak with his family. It cited a US official familiar with Bergdahl's recovery.

In a statement, the Defence Department said it "does not comment on discussions that Sergeant Bergdahl is having with" his medical team.

The Times also reported that Sgt Bergdahl has had no access to news media and is unaware of the controversy raging in the United States about whether the administration put US security at risk by freeing the Taleban inmates.

Mr Kerry said the freed prisoners, who have been released to the Qatari government, are unlikely go back on their word to take up the struggle once more against the US.

"I'm not telling you that they don't have some ability sought some point to go back and get involved. But they also have an ability to get killed doing that," he told CNN.

"I don't think anybody should doubt the capacity of the United States of America to protect Americans." But the decision has garnered criticism, even from Obama's Democratic party.

High-ranking Democratic Senator Dianne Feinstein said "you can't help but worry" about the released militants.

"We have no information on how the United States is actually going to see that they remain in Doha, that they make no comments, that they do no agitations," said Feinstein, the chair of the Senate Intelligence Committee.

She told CBS television's Face The Nation that the Obama administration has not shared any information about possible torture Bergdahl suffered.

"I think this whole sort of deal has been one that the administration has kept very close, and in the eyes of many of us, too close," she said.

There has also been controversy over reports Bergdahl was captured after deserting his post.

In an interview with ABC's This Week, former secretary of state Hillary Clinton said that ultimately, the circumstances of Bergdahl's capture were irrelevant.

The United States always does its best to "bring everybody home off the battlefield," she said.

But Republican Senator John McCain said the prisoner swap was the wrong call.

"The question is... whether it would put the lives of other American men and women who are serving in danger," said Mr McCain, speaking to CNN.

"In my view, clearly, this would," said Mr McCain, a former prisoner of war in Vietnam.

The Times reported that Bergdahl, 28, has emerged from captivity physically in better condition than expected, although he suffers from skin and gum disorders typical of poor hygiene and sun exposure.

Meanwhile, the FBI reported it was investigating threats to his relatives back home in Idaho.

"We are aware of the threats and are working with our local law enforcement partners to investigate," FBI spokesman Jacqueline Maguire told AFP, without providing details as to the nature of the threats.