The Red Cross says it wants to find out why it was not notified about a prisoner held by the US military in Iraq for more than seven months.
Imprisoned Iraqis must be held in line with international law
On Thursday, US Defense Secretary Donald Rumsfeld admitted he ordered the secret detention at the request of CIA director George Tenet.
Under the Geneva Conventions, prisoners should be registered promptly with the Red Cross.
The US was criticised for holding so-called "ghost" detainees in March.
Mr Rumsfeld told a news conference on Thursday that the detainee was a suspected member of the Ansar al-Islam group, which some accuse of a role in the Iraqi insurgency.
When asked why the prisoner was held secretly, Mr Rumsfeld told journalists to ask Mr Tenet and said the letter in which the CIA director had made the request was classified.
He said he did not know how many more prisoners were being held in secret.
Unnamed intelligence officials told news agencies the prisoner had been captured in late June or early July 2003 and was taken to an undisclosed location outside Iraq.
He was then returned to Iraq, they said, where he was held in secret until last month at a high-risk prison near Baghdad, without being listed on any roll or assigned a prisoner number.
Daniel Dellorto, US defence department deputy general counsel, said access by the International Committee of the Red Cross (ICRC) to prisoners could be denied "for purposes of imperative military necessity".
"You need to find information on this person, and the mere availability of this person... for the purpose of seeing the ICRC, might interrupt that or disturb your ability to get information you need to get," said Mr Dellorto.
But the ICRC told BBC News Online that under the Geneva Conventions the US was obliged to register detainees.
"This is of concern to us," said ICRC spokesman Florian Westphal.
"The registration of prisoners makes it possible to do our work... It ensures prisoners do not disappear and that their families are informed of what has happened to them. It is very important."
He said the ICRC would be contacting coalition authorities in Iraq to discuss the circumstances of the unnamed man's detention.
Mr Rumsfeld brushed off suggestions that the man's secret detention sparked concerns he could have been vulnerable to abuse by prison guards.
He insisted the man had been "treated humanely", and had not been held at the Abu Ghraib prison.
In March, a report by Maj Gen Antonio Taguba into serious prisoner abuses at the notorious jail criticised the practice of holding ghost detainees as "deceptive, contrary to army doctrine, and in violation of international law".
According to Mr Rumsfeld, the detention of ghost prisoners is being examined as part of continuing investigations into the abuse scandal.