Rumsfeld said, however, the Pentagon determined it was "appropriate" to not register the prisoner with the International Committee of the Red Cross, but refused to cite the reason, saying it was classified.
The Pentagon acknowledged earlier Thursday that it made a mistake in not registering the alleged senior operative of Ansar al-Islam captured in November and was working to correct the problem.
The United States links Ansar al-Islam to al-Qaeda. "It's broadly understood that people do not have be registered in 15 minutes when they come in," Rumsfeld said.
"What the appropriate period of time is I don't know. It may very well be a lot less than seven months, but it may be a month or more."
The prisoner who has not been named, was held at Camp Cropper, on the outskirts of Baghdad International Airport, officials said.
The New York Times, citing senior defence and intelligence officials, reported in its Thursday edition that Rumsfeld had ordered his detention without notifying the Red Cross to keep it from checking the suspect's treatment and condition.
Senior Pentagon and intelligence officials, have confirmed that the prisoner was hidden along with other "ghost detainees", largely to prevent the International Committee of the Red Cross from monitoring their treatment and conditions.
Rumsfeld's order last November came at the request of George Tenet, the CIA director who resigned this month, according Pentagon spokesman Bryan Whitman.
Both assigning a prisoner number and notifying the Red Cross are required under the Geneva Conventions and other international humanitarian laws.
"I will acknowledge that the ICRC should have been notified about this prisoner earlier," Whitman said. "He will be assigned an identification number and, if appropriate, moved into the general prison population."
Ghost detainees described as "deceptive, contrary to army doctrine, and in violation of international law"
Maj Gen Antonio Taguba,
US army investigator
The report came as the United States continued to conduct a major investigation into the abuse, including sexual humiliation, of prisoners by the US military in both Iraq and Afghanistan.
In March, Major General Antonio Taguba, the army officer who investigated abuses at the Abu Ghraib prison, criticised the practice of allowing ghost detainees as "deceptive, contrary to army doctrine, and in violation of international law".
Pentagon and intelligence officials told the New York Times on Thursday, that the decision to hold the detainee without registering him, at least initially, was in keeping with the administration's legal opinion about the status of those viewed as an active threat in wartime.
"Once he was placed in military custody, people lost track of him," a senior intelligence official told the Times. "The normal review processes that would keep track of him didn't."
Rumsfeld personally ordered the
prisoner be kept unregistered
The detainee was described by the official as someone "who was actively planning operations specifically targeting US forces and interests both inside and outside of Iraq."
The man is still in prison but has only been questioned once while in detention.
The Pentagon's chief spokesman, Lawrence Di Rita, told the paper that officials at Camp Cropper questioned their superiors several times in recent months about what to do with the suspect.
The Times said that only in the last two weeks had a senior Rumsfeld aide asked the CIA to deal with the suspect.