US Defense Secretary Chuck Hagel said Wednesday he had no information suggesting American soldiers were killed in efforts to find Sergeant Bowe Bergdahl, released in a prisoner swap with the Taliban.

"I do not know of a specific circumstance or details of US soldiers dying as a result of efforts to find and rescue Bergdahl," Hagel said after a NATO defence ministers meeting in Brussels.

Pressed for comment amid growing controversy over the exchange and circumstances of Bergdahl's capture five years ago, Hagel said the army had already promised a comprehensive review.

In the meantime, "let us first focus on getting Bergdahl well... getting him reunified with his family," he said, adding it was the military's responsibility to get its soldiers back.

"Other questions... will be dealt with at a later time."

Pressed again for comment, Hagel said any further comment would be "irresponsible."

General Martin Dempsey, the chairman of the US Joint Chiefs of Staff, said Tuesday there would be a full investigation of the case after allegations by other soldiers that Bergdahl might have abandoned his post in 2009.

"Our army's leaders will not look away from misconduct if it occurred," Dempsey said, adding that Bergdahl was innocent until proven guilty.

President Barack Obama has come under fire from Republicans and other critics who say the prisoner swap will encourage others to try to take American soldiers or diplomats hostage.

He has defended the action, saying it was part of his "sacred" trust as commander-in-chief to ensure that no man or woman in uniform is left behind on the battlefield.

State Department deputy spokeswoman Marie Harf revealed that the five Taliban had been among a category of Guantanamo prisoners whose cases were eligible for review.

"Let's say these guys may have eventually been transferred somewhere anyways -- I think many of us would make the argument, I would make it, that we should get something for them," Harf said.

But she quickly dismissed the notion that the other 149 remaining Guantanamo detainees could be viewed by the US as potential bargaining-chips in similar cases.

"Every situation's different. We have a broad goal of closing Guantanamo Bay," she said, saying the current options were either to charge the detainees or try to release them.