The US military has admitted it detained an Afghan man handed over by a US citizen accused of running a freelance counter-terrorism operation.
Jonathan Idema (centre) said he had evidence to prove he worked for the Pentagon
A military spokesman said the prisoner was handed over by the American, Jonathan K Idema, in May.
A BBC correspondent in Kabul says that the disclosure is embarrassing for the US, which said it had had no links with the alleged American mercenary.
He is facing charges of torture, kidnapping and running a private jail.
"We did receive a detainee from Mr Idema or his party," said Major Jon Siepmann, spokesman for the coalition forces.
"The reason we received this person was that we believed that he was someone that we had identified as a potential terrorist and we wanted him for questioning," he said.
But forces strenuously deny that Mr Idema was working for the military in any official capacity and insist that he is a mercenary.
They argue that they were not aware of Mr Idema's "full track record" prior to his arrest earlier this month along with two other Americans and four Afghans.
Eight prisoners were freed from a makeshift jail in Kabul they are alleged to have run.
Mr Idema and his co-accused argue that they have been abandoned by the US authorities
Mr Idema argues that he was working with the knowledge of the US defence secretary, and that the US government had abandoned him.
He said that in May he had handed over a man to the US authorities who he described as a Taleban intelligence chief.
The suspect was released a month later when US officials decided he was not who Mr Idema said he was.
The American went on trial on Wednesday with two other compatriots, Edward Caraballo and Brent Bennett.
They faced charges of torture, kidnapping and running a private jail.
Speaking to journalists before Wednesday's court proceedings, Mr Idema said he had evidence to prove he was working for the Pentagon.
He claimed to have helped prevent several attempted terrorist attacks and said he had regular e-mail, phone and fax contact with Defence Secretary Donald Rumsfeld's office and other senior Pentagon officials.
He said his group had prevented assassination attempts on Education Minister Yunis Qanooni and Defence Minister Marshal Mohammad Qasim Fahim.
The BBC correspondent in Kabul says that while Mr Idema's claims are being treated cautiously, many feel that more details will emerge with every new development in this saga.
Four Afghans arrested with the Americans also stood for trial on Wednesday.
After the charges were read out, a lawyer for one of the Americans - Mr Caraballo - asked for the trial to be delayed by at least two weeks so the defence could be better prepared.
Presiding Judge Abdul Baset Bakhtyari allowed the request and adjourned the case for 15 days.
None of the Americans spoke officially in court on Wednesday but three witnesses did appear.
They were from among the eight Afghans who were found at the alleged private jail the Americans were said to be running.
One, Sher Jan, said: "They pulled me out of my house one morning, hooded me and broke a rib with a gun... They poured hot water on me too."
Correspondents say the US bounty for al-Qaeda fugitives has drawn many foreign vigilantes to Afghanistan.
The US government has promised $25m for anyone who facilitates the arrest of al-Qaeda leader Osama Bin Laden.