President Barack Obama Tuesday defended a prisoner swap with the Taliban to free an American soldier, saying his "sacred" trust as commander-in-chief outweighed claims he broke the law and set a dangerous precedent.

Obama is facing rising questions about the deal to secure the release of US Army Sergeant Bowe Bergdahl after five years, in return for the transfer of five Taliban prisoners from the "war on terror" prison at Guantanamo Bay.

At a press conference in Poland, Obama was unrepentant, as the White House issued a new legal justification for his action.

"The United States has always had a pretty sacred rule. That is we don't leave our men or women in uniform behind," Obama said.

"We saw an opportunity, we were concerned about Sergeant Bergdahl's health... and we seized that opportunity," Obama said.

Initial euphoria in Washington about Bergdahl's release has degenerated into a mounting political row, shaped by consistent Republican claims that Obama is a naive leader and a liability as commander-in-chief.

It has also revived a showdown over Obama's still unfulfilled vow to close down Guantanamo Bay -- which has repeatedly been blocked by Congress.

The president's decision to sign off on the swap has also been criticized because of reports that Bergdahl deserted his post in Afghanistan nearly five years ago.

Obama said Bergdahl's state of health had not yet permitted an interrogation about his capture -- but said the mystery of his disappearance didn't change his own basic obligation as commander-in-chief.

"We still get an American soldier back if he is held in captivity. Period. Full stop. We don't condition that."

Lawmakers have complained that they were not given the 30 days notice required by law ahead of prisoners being transferred out of the war on terror camp at Guantanamo Bay, Cuba.

"The process was truncated because we wanted to make sure that we did not miss that window," Obama said.

But he added that his administration had been talking with Congress "for quite some time" about the idea that a prisoner swap may be required to win Bergdahl's release.

On Tuesday it was announced that the White House had apologised for keeping lawmakers in the dark regarding the swap.

Amid growing concern among Republicans and Democrats, Senate Intelligence Committee chair Diane Feinstein voiced her disappointment at not having been told about the deal in advance.

She said National Deputy Security Advisor Tony Blinken had called her Monday night.

"He apologized for it and said it was an oversight," Feinstein said, according to National Journal.

Bergdahl -- the only US soldier held by the Taliban after being captured in Afghanistan -- was freed on Saturday in a dramatic deal brokered by Qatar.

In exchange, five Taliban prisoners were turned over to the Arab emirate where they will remain for a year.

But some Republicans warned they could return to the battlefield and pose a threat to Americans abroad.

Senator John McCain said he was worried Washington gave up "war criminals" in exchange for Bergdahl.

Robert Menendez, Democratic chair of the Senate Foreign Relations Committee said he was "concerned about who was given in exchange and I am concerned about what precedents we set here for exchanges".

Obama said Qatar had set up a process to monitor the released Islamist prisoners and that the United States would be "keeping eyes on them."

But he admitted it was possible that some of them could return to activity "detrimental" to the United States.

"I would not be doing it if I thought it was contrary to American national security," Obama said.

"This is what happens at the end of wars. At some point, you try to make sure you get your folks back."

- 'Innocent until proven guilty' -

The Chairman of the Joint Chiefs of Staff, General Martin Dempsey also weighed in on the transfer, saying it was likely the "last, best hope" to free Bergdahl.

"The questions about this particular soldier's conduct are separate from our effort to recover ANY US service member in enemy captivity," Dempsey wrote on Facebook.

"As for the circumstances of his capture, when he is able to provide them, we'll learn the facts. Like any American, he is innocent until proven guilty."

Dempsey also warned that the Army would not shirk from misconduct charges against Bergdahl, if merited.

Obama's National Security Council defended the legal justification for the swap.

NSC spokeswoman Caitlin Hayden said Defence Secretary Chuck Hagel had used his power under US law to transfer prisoners from Guantanamo after ruling that doing so would enhance US national security.

Hagel also determined that offering the full 30-day notice to Congress "could endanger the soldier's life," she said.

Bergdahl's almost five years in captivity saw him transferred between various militant factions along the volatile Afghanistan-Pakistan border.

He is now being treated at a US military facility in Germany.