The 31-year-old, who disappeared from a remote combat outpost in the summer of 2009 and was called “a traitor” by Donald Trump, also pleaded guilty to a charge of misbehaviour before the enemy.
Bergdahl’s lawyer at the military hearing at Fort Bragg, North Carolina, said the defence and prosecution had not been able to agree to a so-called stipulation of facts in the case.
The Associated Press said this was a likely indication that they had been unable to come to an agreement about a limit to his term of punishment.
Bergdahl, from Idaho, was held for five years by the Haqqani network, the same militant faction linked to the Taliban, that held American Caitlan Coleman and her family hostage for five years until being freed last Wednesday. After enduring what one US official said was intense abuse while in captivity, he was freed as part of a controversial prisoner swap.
He was released in 2014 in exchange for five Taliban prisoners who had been detained at Guantanamo Bay. While campaigning for the White House, Mr Trump called Bergdahl “garbage” and suggested that he should have been summarily executed.
“You know in the old days - Bing. Bong,” Mr Trump said in one speech, as he he mimicked the firing of a rifle. “When we were strong.”
During his hearing on Monday before Army Col Jeffery Nance, Bergdahl said: “I left my observation post on my own. I understand leaving was against the law.”
The charge of misbehaviour before the enemy carries a maximum charge of life in prison, though the judge will have to take many factors into account when he starts the sentencing process, scheduled to begin next week.
Former members of Bergdahl’s unit have claimed that the deaths of six US soldiers have been tied, either directly or indirectly, to the search for him after he disappeared, according to CNN. The New York Times said prosecutors had argued once he disappeared, his unit was obliged to change its operations and said two soldiers and a Navy SEAL suffered serious injures in search operations.
However, lawyers for Bergdahl have said those injuries cannot be directly tied to the accused. Military investigators later found serious errors with the planning of one search mission.
Bergdahl had raised the issue of whether he would be able to obtain a fair trial given Mr Trump’s comments. In an interview last year with British filmmaker Sean Langan, who was himself held captive by the same Taliban group in 2008, Bergdahl said Mr Trump’s comments would have had a serious impact.
“We may as well go back to kangaroo courts and lynch mobs that got what they wanted,” Bergdahl said in the interview, broadcast by ABC. “The people who want to hang me, you’re never going to convince those people.”
Bergdahl has been assigned to desk duty at a Texas Army base while his case unfolds.