The main US political parties have clashed over the deal to swap five Guantanamo Bay detainees for a Taliban-held soldier, with Republicans warning it could put American lives at risk.
Senator John McCain said the detainees, who were transferred to Qatar, were some of the “highest high-risk people”.
Afghanistan also attacked the deal, saying handing prisoners to a third country was against international law.
Sgt Bowe Bergdahl, 28, was handed to US forces in Afghanistan on Saturday.
In an emotional address on Sunday, his father, Robert Bergdahl, said he was proud how far his son was willing to go to help the Afghan people, but warned that his recovery would take a long time.
He said he and his wife had not yet spoken to the soldier, who is in a good condition and currently undergoing medical care at a US military hospital in Germany.Legality
Several Republicans have spoken out against the deal, warning that it set a worrying precedent and amounted to negotiating with terrorists.
Mr McCain said the Taliban released were “possibly responsible for the deaths of thousands” and may have “the ability to re-enter the fight”, in comments to CBS TV.
Republican chairman of the House intelligence committee, Mike Rogers, told CNN that Washington had “now set a price” for al-Qaeda ransom threats.
Chuck Hagel: “No shots were fired – it went as well as it could have”
Republican representative Adam Kinzinger said he would celebrate Sgt Bergdahl’s return but called the release of the Taliban men “shocking”.
Questions were raised over the legality of the deal, after the Obama administration did not give Congress sufficient notice about the transfer of the Taliban detainees.
But US Defence Secretary Chuck Hagel, who is currently in Afghanistan, was quick to dismiss allegations of wrongdoing, saying the military had to act quickly “to essentially save his life”.
“We didn’t negotiate with terrorists… As I said and explained before, Sergeant Bergdahl was a prisoner of war. That’s a normal process in getting your prisoners back,” he told NBC TV.
US National Security Adviser Susan Rice said that Sgt Bergdahl’s failing health had created an “acute urgency” to act and therefore made it “necessary and appropriate” not to adhere to the 30-day notification requirement.
The Afghan government, which was not informed of the deal until after the exchange had taken place, condemned it as a “breach of international law” and urged the US and Qatar to “let the men go free”.
The five detainees are thought to be the most senior Afghans held at the US detention facility in Cuba, having been captured during America’smilitary campaign in 2001.
In a rare public statement on Sunday, Afghan Taliban leader Mullah Mohammad Omar described the exchange as a “big victory”.
But President Barack Obama said that he had received security guarantees from Qatar – which mediated the deal – “that it will put in place measures to protect our national security”.
They have been banned from leaving Qatar for at least a year.
Sgt Bergdahl, of Hailey, Idaho, was the only US soldier being held by the Taliban in Afghanistan. He was serving with an infantry regiment in Paktika province, near the Pakistani border, when he went missing on 30 June 2009.
“Bowe’s been gone for so long that it’s going to be difficult to come back,” his father told journalists on Sunday.
“It’s like a diver going deep on a dive and [having] to stage back up through decompression to get the nitrogen bubbles out of his system. If he comes up too fast, it could kill him.”
The circumstances of Sgt Bergdahl’s capture remain unclear, with speculation he may have walked away from his base out of disillusionment with the US campaign.
US officials say any decision over possible desertion charges will be made by the army, but there is a feeling the soldier has suffered enough.