Secretary of State John Kerry is defending the swap of five Guantanamo detainees for American Bowe Bergdahl, amid reports the captive US soldier was kept in a metal cage and total darkness.

Kerry on Sunday doubled down on President Barack Obama’s controversial decision to make the trade in exchange for the release of Bergdahl, who the US diplomat said was at risk of torture.

“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.

Sgt Bowe Bergdahl in captivity.

Bergdahl was released last Saturday to American troops in Afghanistan in exchange for the five Taliban detainees held at Guantanamo Bay, Cuba.

Kerry made his remarks amid a news report on Sunday about the grim conditions Bergdahl was held in while a hostage in Afghanistan.

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

Bergdahl, who is receiving medical 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 briefed on his condition.

“Physically, he could be put on a plane to the US tomorrow but there are still a couple of mental criteria to address: the family unification piece and the media exposure piece,” said one US official who has been briefed on his condition told the Times.

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

Kerry said the freed Taliban, who have been released to the government of Qatar, are unlikely go back on their word and take up the struggle again against the US.

Sgt. Bowe Bergdahl in front of the American flag. Photo: Getty

Meanwhile the FBI says it is investigating threats to Bergdahl’s relatives in Idaho.

“We are aware of the threats and are working with our local law enforcement partners to investigate,” spokeswoman Jacqueline Maguire said on Sunday.

“As always, we take these types of threats seriously,” she said.