Civilian attorney David E. Coombs' statement deflates an effort by Amnesty International to involve the British government in a dispute over Pfc. Bradley Manning's pretrial confinement in a Marine Corps brig in Quantico, Va.
Because Manning's mother is Welsh, some had speculated that the Oklahoma-born soldier held dual citizenship. The London-based human-rights group had asked British authorities on Tuesday to intervene should it be established that Manning is British.
In a statement e-mailed to The Associated Press, Coombs wrote: "Pfc. Bradley Manning does not hold dual citizenship. He is an American, and is proud to be serving in the United States Army."
Amnesty International didn't immediately respond to an AP query about Coombs' statement.
Manning is charged with illegally downloading classified documents and video while working as an intelligence analyst in Iraq and giving it to an unauthorized person. The video appeared on WikiLeaks.
Military officials say Manning is the suspected source of tens of thousands of classified government documents WikiLeaks has published since July.
Amnesty International has called Manning's confinement repressive and inhumane. His maximum-security classification keeps him confined to his single-bed cell 23 hours a day. He is also on "prevention of injury" watch, which bars him from keeping personal items in his cell or having sheets and a pillow. Instead, he sleeps beneath a blanket on a mattress with a built-in pillow shape. Manning also must respond to a guard's queries every five minutes when he is awake.
Coombs has filed a formal complaint with the Quantico commander saying the strict conditions lack medical justification and violate military rules. He has asked to have Manning reclassified as medium security, which would allow him to keep personal items in his cell, move about outside his cell without restraints, perform menial jobs within the brig and possibly move to dormitory quarters.
Coombs said in his statement that "it is a basic fundamental right not to be unlawfully punished prior to trial."
The United Nations special rapporteur on torture is looking into Manning's confinement in response to complaints from Manning supporters.
Military officials say Manning's security status is based on the Navy Corrections Manual, which mandates "maximum custody" for prisoners who pose a high probability of escape, are potentially dangerous or violent and whose escape would cause concern of a threat to national security.
Quantico spokesman Col. Thomas V. Johnson said international attention alone won't lead to changes in Manning's treatment.
"Until someone in the chain of command tells the Marine Corps to stop doing what you're doing, we continue to carry out our mission, which is, in this case, to provide for his detention," Johnson said.
Associated Press reporter Raphael Satter in London contributed to this story.