The soldier who was returned to US forces in exchange for the release of five Taliban operatives looked sick and drugged in a proof-of-life video, US senators said Wednesday.

Lawmakers emerging from a classified briefing by Pentagon personnel said US officials negotiating the release of Sergeant Bowe Bergdahl had demanded the video from his captors as a condition for beginning talks on last weekend's prisoner-of-war swap.

"He didn't look good. I understand the emotional power that that video would have had on the president," Senate Republican Mark Kirk told reporters.

He said Bergdahl spoke in stammering English, and provided proof of life facts such as the death of Nelson Mandela last December, meaning the video was at most six months old.

Senator Dick Durbin added that the video "made clear that this man was not in good condition," a situation he said may have prompted the call for immediate action by the White House to rescue the soldier who had been held captive by the Taliban in Afghanistan since 2009.

"He looked either drugged or tired or sick," Durbin said. "It's hard to describe, but he did not look like a normal person."

President Barack Obama has described Bergdahl's deteriorating health and an imminent danger risk as justifications for the hasty rescue.

But while Democrat Joe Manchin agreed that Bergdahl had looked drugged, he opposed the idea that the soldier's poor condition justified the White House taking swift action and avoiding consultations with Congress before the swap was made.

"His health was not the critical factor," Manchin said.

With concern mounting over how the prisoner swap was carried out, more senators expressed anger that five hardened extremists were released from Guantanamo to Qatar, where authorities are required to hold the men for one year.

"I'm increasingly skeptical that this was a good trade for America," Republican Senator Marco Rubio said.

"We've also released five very dangerous individuals who I believe will rejoin the fight against America. And we've set a precedent that now will encourage other enemies of the United States to seek out to try and capture American men and women in uniform."

Senator John McCain, himself a former POW in Vietnam, blasted the swap as a disaster for US national security.

"I promise you, in a year from now, if not before, they will be back in Afghanistan and in the fight," the Arizona Republican said.

Critics have also zeroed in on the controversial circumstances of Bergdahl's capture.

He left his post at a forward operating base in Afghanistan, and fellow soldiers have come forward to accuse him of being a deserter.

Asked if briefers provided any evidence that Bergdahl deserted the military, Manchin said "there's not overwhelming information he didn't."