Ashcroft, on a visit to Norway, said on Friday the 660 prisoners held in Guantanamo Bay were treated with "respect for their humanity", but signalled they could be held indefinitely.
"When you detain unlawful combatants in time of war, generally throughout history there has been the capacity to maintain those individuals as detainees pending the outcome of the conflict," he said.
"I will concede this is a different and unique kind of conflict. But it's pretty clear that the war is very active."
War on Terror
He added: "Terrorism is rearing its ugly and vicious head around the world on a recurrent basis."
"We are going to continue to fight the war against terror effectively and aggressively."
Norwegian Justice Minister Odd Einar Doerum said he had urged Ashcroft to step up legal protection for the prisoners - called "detainees" by the Pentagon.
Ashcroft calls the Guantanamo
prisoners ''unlawful combatants''
"They must be looked upon as prisoners of war until their status is decided upon by a tribunal which is accepted in accordance with the Third Geneva Convention," he said.
The convention lays out terms for humane treatment of prisoners, states trials "shall take place as soon as possible", and that prisoners should get legal aid.
But Ashcroft said the detainees were "unlawful combatants" who did not qualify as prisoners of war because they "attacked innocent civilians without warning", and operated without uniforms.
Th US has faced international condemnation over its treatment of the prisoners who were airlifted to Cuba after the 2001 Afghan war.
More than 40 detainees have been released since then, all of whom were in the wrong place at the wrong time.