Congress was deliberately kept out of the loop regarding negotiations to free a US soldier because his Taliban captors threatened to kill him if the news leaked, a Senate aide said Thursday.

Army Sergeant Bowe Bergdahl was handed over to US forces in Afghanistan last weekend in exchange for the transfer of five senior Taliban leaders held by the United States.

The swap was immediately criticized by lawmakers who said the White House did not give members of Congress the required 30 days notice before transferring detainees from Guantanamo.

"Senators were informed at the briefing yesterday that the US obtained credible information that, if anything about the swap became public, Bergdahl would be killed," a Senate aide told AFP.

On Wednesday, Obama administration officials briefed the Senate's 100 members, showing them a proof-of-life video made by Bergdahl's captors and providing details on the swap.

A senior US official said the senators were told that Bergdahl's life "could be jeopardized if the detainee exchange proceedings were disclosed or derailed."

Some senators said Bergdahl looked sick or drugged in the footage, which could support President Barack Obama's argument that the soldier's deteriorating health was a top consideration in rushing the swap.

Other senators said they believe Bergdahl's health was not the critical issue.

But Democrats, including Senate Intelligence Committee chair Dianne Feinstein, were left fuming at being kept uninformed, with Feinstein warning that Obama broke the law by not telling Congress ahead of the swap.

She said a White House official called her Monday to apologize for not informing her earlier.

The Senate's top Democrat Harry Reid was informed on Friday, the day before the prison exchange, while House Speaker John Boehner and Senate Minority Leader Mitch McConnell, both Republicans, were told on Saturday.

Questioned about whether Obama should have alerted lawmakers sooner, Reid shot back: "Is it Friday or Saturday? What difference does it make?"

Reid, without mentioning the threat to Bergdahl's life, pointed to the president's narrow window in which to act.

Obama, he said, "had a decision to make. He had an opportunity to bring home an American soldier, and he brought him home, and I'm glad he did."

But the exchange has blown up into another clash between the White House and Congress, with lawmakers warning that releasing five battle-hardened Taliban operators, two of whom stand accused of mass killings, was a disastrous result that endangers Americans.

Concerns about Bergdahl himself have emerged, with some soldiers in his unit saying they believe he had deserted his post in June 2009 before his capture.

A classified military report on the investigation into Bergdahl's disappearance said he had wandered away from his post on a previous occasion but had returned, people briefed on the report told The New York Times.