Freed US soldier Bowe Bergdahl developed a love for Afghan green tea, taught his captors badminton, and even celebrated Christmas and Easter with the hardline Islamists, a Pakistani militant commander told AFP Sunday.

Bergdahl, the only US soldier detained in Afghanistan since war began in 2001, was released Saturday in exchange for the freeing of five senior Taliban figures held at Guantanamo Bay, in a dramatic deal brokered by Qatar.

The army sergeant's almost five years in captivity saw him transferred between various militant factions along the volatile Afghanistan-Pakistan border, finally ending up in Pakistan's North Waziristan tribal district, according to militant sources.

A commander of the Haqqani network, a militant outfit allied with the Taliban with ties to Al-Qaeda, on Sunday painted a picture of a man who adjusted to his new life by engaging with his captors while clinging to aspects of his own identity.

"He was fond of kawa (Afghan green tea). He drank a lot of kawa all day, which he mostly prepared himself," the commander told AFP by phone from an undisclosed location in Pakistan's tribal areas.

Over time, Bergdahl, now 28, grew fluent in Pashto and Dari, he said.

Unlike the militants, who were mainly ethnic Pashtuns known for their voracious appetite for meat, Bergdahl "liked vegetables and asked for meat only once or twice a week", the commander said.

While the militants attempted to teach the soldier about Islam and provided him with religious books, he preferred more earthly pursuits.

"He would spend more time playing badminton or helping with cooking," the militant chief said.

"He loved badminton and always played badminton with his handlers. In fact, he taught many fighters about the game," he added.

And the Idaho native made a point of celebrating the Christian festivals he was accustomed to back home, even inviting his captors to participate.

"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," he said.

Imtiaz Gul, a security analyst, said the militants would have regarded Bergdahl as a high-value asset and harming him would have had a negative impact on their propaganda efforts.

"These groups usually treat hostages that way," he said.

- Mystery still surrounds capture -

The insights into Bergdahl's life are the clearest to emerge since he was captured in eastern Afghanistan in June 2009 and appeared in a Taliban video a month later.

"I was captured outside of the base camp. I was behind a patrol, lagging behind the patrol and I was captured," Bergdahl said in the video, later growing distraught when discussing his family.

According to the commander, Bergdahl then came under the custody of the late Mullah Sangeen Zadran, a key leader in the Haqqani network.

An Afghan Taliban source added that Bergdahl fell into the group's hands after initially being captured by a criminal outfit linked to the Taliban.

Militant sources disagree over the circumstances surrounding his capture, but several -- then and now -- described him as being "drunk".

The US military has never commented on the issue.

US Defense Secretary Chuck Hagel demurred Sunday when asked by reporters 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".

"Sangeen kept him in Paktika, Paktia and parts of Khost before bringing him to the Pakistan-Afghanistan border," the Haqqani commander said, referring to districts located in eastern Afghanistan.

Zadran was killed by a US drone strike in September 2013, after which Bergdahl was sent on to the Pakistani tribal zone of North Waziristan, where the feared Haqqani network -- known for their spectacular attacks on foreign forces -- are headquartered.

Following his capture, Bergdahl went on to appear in several more videos, sometimes appearing gaunt and taking a hostile line against the US-led war effort.

Attention is now likely to focus on whether he was coerced into making those statements as well as unravelling the mysterious nature of his capture.

- Operation to 'save his life' -

The operation to free Bergdahl was launched after intelligence showed that his health had deteriorated, Hagel said.

"We believed that the information we had ... was such that Sergeant Bergdahl's safety and health were both in jeopardy," he told reporters Sunday.

"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."

Some specifics of the operation "are classified and will remain that way", Hagel said.

"Fortunately, as you know, no shots were fired," he told reporters of the handover, according to a Pentagon transcript. "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."

Bergdahl will now remain at the Landstuhl centre in Germany while he continues his "reintegration process," the army said. Officials had said Saturday he was in "good" condition.

"This is a guy who probably went through hell for the last five years," Hagel told NBC's "Meet the Press".

"Let's focus on getting him well and getting him back with his family."