Secretary of State John Kerry defended 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.

Kerry doubled down on President Barack Obama's controversial decision to release five Taliban 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," Kerry told CNN's "State of the Union" program.

Kerry made the remarks -- his first on the controversial prisoner exchange -- amid a news report about the grim conditions in which 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.

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 Defense Department said it "does not comment on discussions that Sergeant Bergdahl is having with" his medical team.

The Times also reported that 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 Taliban inmates.

- 'You can't help but worry' -

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 McCain, speaking to CNN.

"In my view, clearly, this would," said 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 spokeswoman Jacqueline Maguire told AFP, without providing details as to the nature of the threats.

US advisor defends remarks on freed soldier, Benghazi
Washington (AFP) June 07, 2014 - US National Security Advisor Susan Rice is defending her comments that Army Sergeant Bowe Bergdahl, freed in a prisoner swap after five years' captivity with the Taliban, had served "with honor."

Her remarks have come under fire amid allegations that Bergdahl was captured in Afghanistan only after deserting his post.

Rice, who first spoke on a US talk show last weekend shortly after Bergdahl was turned over to US special forces, said it was too soon to judge the soldier's actions.

"This is a young man whose circumstances we are still learning about. He is, as all Americans, innocent until proven guilty," Rice told CNN on Friday.

"Let the military work in the first instance to bring him back to health. We will have a full, comprehensive review of what happened and then we will be able to" determine whether he was a deserter, she said.

Rice said that regardless, her description of Bergdahl as a man who served "with honor and distinction" was correct.

"What I was referring to is the fact that this was a young man that volunteered to serve his country in uniform at a time of war. That is, itself, a very honorable thing," she said.

The uproar over Rice's Bergdahl comments is drawing parallels to another controversy triggered by remarks she made soon after the assault at the US mission in Benghazi, Libya.

Serving at the time as US envoy to the United Nations, Rice initially said the attack was fueled by local anger over an anti-Muslim video posted on YouTube.

The comments sparked a firestorm of criticism, especially among Republicans.

It has since become clear that the September 11, 2012 attack on the mission -- which cost the lives of four Americans, including ambassador Chris Stevens -- was planned by armed militants.

But Rice said she works to be "upfront with the American people."

On Benghazi, she said, "I provided the best information the US government had at the time. Parts of it turned out to be wrong. I regret the information I was provided was wrong."

"That doesn't make me a liar," Rice added.

"That makes me a public servant trying to say what we knew at the time."