Amnesty International called on British authorities to intervene Tuesday in the case of the Army private accused of leaking material to the secret-spilling website WikiLeaks amid claims that he is an American-British dual national.

Pfc. Bradley Manning has been held at the Marine Corps brig in Quantico, Virginia since last summer under conditions which his supporters describe as punitive. Manning's case has attracted sustained attention in Britain in part because his mother is Welsh, but some supporters now claim that the 23-year-old holds British citizenship.

In a statement e-mailed to The Associated Press, Amnesty International's U.K. Director Kate Allen said Manning's background meant that British officials "should be demanding that the conditions of his detention are in line with international standards."

Amnesty spokesman Neil Durkin cautioned that that did not mean the rights group necessarily accepted that Manning was a U.K. citizen — merely that it believed the U.K. government had a duty to get involved "should it be established that he is British."

Manning was born in Oklahoma but he moved to Wales when his parents separated.

His mother was born in the Welsh town of Haverfordwest in 1953, according to government records cited by the Guardian newspaper Tuesday. The paper quoted Alison Harvey, the head of the Immigration Law Practitioners' Association in London, as saying that "that makes Bradley Manning British."

Britain's Foreign Office declined comment, saying it couldn't release information on an individual's nationality without their consent. An e-mail and a phone call seeking comment from Manning's lawyer David E. Coombs were not immediately returned.

On his website, Coombs says Manning is being kept in solitary confinement 23 hours a day, barred from having sheets or using a pillow, prohibited from keeping personal items in his cell, and forced to respond to a guard's queries every five minutes. Manning cannot exercise while in his cell, and he must strip down to his underwear before going to bed. If guards can't see him clearly when he's sleeping, he is woken.

Manning's case — and the restrictive conditions under which he is being kept — have become the focus of international attention following WikiLeaks' controversial publication of hundreds of thousands of secret U.S. intelligence and diplomatic documents. Manning is suspected of being the source of the leaked documents, although the charges against him are narrower.

Brig officials claim Manning hasn't been treated differently from any other inmate. But Coombs disputes that, noting that, unlike other prisoners, Manning is being held as a maximum security prisoner and subjected to "prevention of injury watch" against the advice of psychiatrists.