US Defense Secretary Chuck Hagel said Sunday an operation to free soldier Bowe Bergdahl in exchange for Taliban prisoners was undertaken to save his life, as the insurgents' leader hailed the swap as a "big victory".

Bergdahl, the only US soldier captured by the Taliban since the war began in 2001, was freed after almost five years in captivity in exchange for five senior insurgents who had been detained at the US military prison in Guantanamo Bay.

Both parties claimed success for the dramatic deal, which was brokered by Qatar. Hagel even suggested it could provide a breakthrough for peace in Afghanistan.

But the swap was criticised by Republican lawmakers, who demanded to know whether the Taliban prisoners would return to fighting the United States and questioned whether other Americans abroad would be at risk.

Hagel provided new details on the operation to free the 28-year-old Bergdahl to reporters while en route to Bagram air base north of Kabul, where he paid a brief, unannounced visit to meet with more than a dozen members of the team that retrieved Bergdahl.

"We believed that the information we had, the intelligence we had, was such that Sergeant Bergdahl's safety and health were both in jeopardy, and particularly his health deteriorating," Hagel said, according to a Pentagon transcript.

"It was our judgement that if we could find an opening and move very quickly with that opening, that we needed to get him out of there, essentially to save his life."

- 'No shots fired' -

Dozens of US special forces troops backed up by helicopters were sent to an undisclosed location in eastern Afghanistan where Taliban militants handed over Bergdahl, defence officials have said.

"Fortunately, no shots were fired," Hagel explained. "There was no violence. It went as well as we not only had expected and planned, but I think as well as it could have."

In an interview with NBC's "Meet the Press" from Bagram, Hagel said the prisoner trade could provide a window of opportunity for peace in Afghanistan.

"Maybe this will be a new opening that can produce an agreement," he said, noting that the United States had engaged in talks with the Taliban in the past.

Mullah Mohammad Omar, the spiritual leader of the Taliban, issued a rare statement praising the release of the five Guantanamo detainees as a "big victory," in what some saw as a potential confidence-building measure.

Bergdahl's release came four days after President Barack Obama announced a timetable for a final US pullout by end-2016.

- Insurgents sent to Qatar -

The five Guantanamo prisoners were named as Mohammad Fazl, Norullah Noori, Mohammed Nabi, Khairullah Khairkhwa and Abdul Haq Wasiq -- all influential former officials of the Taliban regime toppled by the US-led invasion of Afghanistan after the 9/11 attacks.

They were swapped for Idaho native Bergdahl, who disappeared from a base in Afghanistan's eastern Paktika province in 2009.

US officials confirmed that the United States had transferred the five Afghan Guantanamo detainees to Qatar, as did a Taliban statement which said the men were with their families.

A Qatari source said the Taliban detainees would spend one year in the Gulf state, while Kabul called for their immediate release, calling the transfer illegal.

The men's release had long been the main condition imposed by the Taliban to launch peace negotiations with the United States.

"I think it shows all sides' goodwill for trust-building and start of the peace talks in near future," Ismail Qasimyar, of Afghanistan's High Peace Council, said of the deal.

- Bergdahl 'never missed' Christmas -

A Pakistani militant commander told AFP that Bergdahl developed a love for Afghan green tea, taught his captors badminton, and even celebrated Christmas and Easter with the hardline Islamists during his time in captivity.

"He never missed his religious festivals. He used to tell his handlers they were coming up weeks before Christmas and Easter and celebrated it with them," the commander said.

Bergdahl's parents held a tearful press conference in Idaho on Sunday in which they revealed they have yet to speak to their son.

"Bowe has been gone so long that it's going to be very difficult to come back," his father Bob said, likening the process to a diver resurfacing slowly. "If he comes up too fast, it could kill him."

Both mother and father addressed their son directly in the press conference, however, telling him they loved him and urging him to trust the people around him.

Bergdahl arrived Sunday at the Landstuhl centre in southern Germany where he was to continue his "reintegration process," the army said.

Officials initially said the soldier was in "good" condition and able to walk on his own.

The circumstances of his disappearance remain unclear.

Hagel demurred when asked if Bergdahl had gone AWOL (absent without leave) or deserted his post, saying only that "other circumstances that may develop, and questions -- those will be dealt with later".

"This is a guy who probably went through hell for the last five years," Hagel told NBC. "Let's focus on getting him well and getting him back with his family."

While Bergdahl's release was met with joy in the United States, several Republican lawmakers questioned the long-term costs.

Influential Senator John McCain called the Taliban figures the "hardest of the hard core" and noted: "It is disturbing that these individuals would have the ability to re-enter the fight."