Relief at the release of the only US soldier taken prisoner in the Afghan war gave way on Monday to concern and in some cases anger over the price paid for his freedom.

Five years after his capture, Sergeant Bowe Bergdahl was freed on Saturday after Washington agreed to hand five senior Taliban prisoners into the custody of Qatar, which brokered the deal.

The agreement was warmly welcomed by US President Barack Obama and the 28-year-old soldier's parents at an emotional White House ceremony.

The Taliban's supreme leader Mullah Mohammad Omar also hailed the prisoner swap, thanking Qatar and calling the release of his comrades a "big victory" for his movement.

But the deal quickly sparked controversy.

In Washington, Obama came under fire for failing to consult Congress about a deal some Republican lawmakers said was both illegal and a gift to America's Taliban enemy.

And in Afghanistan, many expressed disgust that Taliban figures accused of some of the bloodiest crimes in the country's civil war had been freed in exchange for a single US soldier.

- Deserting his post? -

Several senior US lawmakers accused the White House of having negotiated and cut a deal with "terrorists," and warned that it would put American troops in the field at even greater risk.

And Bergdahl himself was criticized in US media by some of his former comrades in arms, who alleged he had only been captured after deserting his post.

Writing in the Daily Beast, a member of Bergdahl's unit -- the 1st Battalion of the 501st Infantry -- accused him of putting fellow soldiers at risk by his actions.

"Every member of my brigade combat team received an order that we were not allowed to discuss what happened to Bergdahl for fear of endangering him," Nathan Bradley Bethea wrote.

"He is safe, and now it is time to speak the truth. And that the truth is: Bergdahl was a deserter, and soldiers from his own unit died trying to track him down."

The Taliban detainees -- Mohammad Fazl, Norullah Noori, Mohammed Nabi, Khairullah Khairkhwa, and Abdul Haq Wasiq -- were transferred from Guantanamo Bay to Qatar.

The gas-rich Gulf emirate is a US ally but maintain links to the Taliban, which operates a Qatar office as a mouthpiece to the outside world.

Of the five, Noori, former governor of the northern province of Balkh, is seen as the most controversial. In his former fief, he is accused of taking part in the 1998 genocide of thousands of Shiites.

"Our close relatives and loved ones and neighbors were killed during Noori's term as governor," Hussain Ali, a taxi driver in the Balkh provincial capital of Mazar-i-Sharif, told AFP.

The allegations around Noori center on an August 1998 massacre of up to 8,000 Shiites at the hands of the Taliban, who in turn had set out to avenge the killing of 2,000 of their own men the year earlier.

Afghan President Hamid Karzai has not officially commented but his government has criticized the release as "illegal" for transferring the men to a third country.

Outside Balkh, in Pashtun-dominated parts of the south of the country, there was more understanding of the release. A senior government negotiator said it could help in the search for peace.

- Prisoner of war -

"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 Sunday.

White House and State Department officials insisted Bergdahl's status as a prisoner of war was a special one and that the deal did not herald a new policy of US negotiations with terrorists.

State Department spokeswoman Jen Psaki said there had been an "opportunity to save his life" which arose "only in the last week."

"We took steps needed to secure the return and release of a prisoner of war who was a member of the military, and that's why we made the decisions we did," she said.

When asked if he was a deserter, Psaki replied: "We would characterize him as a member of the military who was detained while in combat."

On Sunday, Bergdahl arrived at the US military medical center in southern Germany.

A Pentagon spokesman said Monday that he was in stable condition and required continued hospitalization, in part to address "dietary and nutritional needs."

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