A British diplomat "called the State Department on the 29th of March to draw attention to our concerns over Bradley Manning," the British embassy spokeswoman said, adding that the Foreign Office expected to "follow up" on the case.

Manning's detention conditions, which include solitary confinement and being forced to sleep naked, have drawn the attention of Amnesty International, the American Civil Liberties Union and others including a Welsh member of the British parliament, Ann Clwyd.

Amnesty International in February called Manning's detention "harsh and punitive" and urged Britain to intervene on his behalf.

Manning, 23, was arrested last year while deployed to Iraq amid suspicions he had passed to WikiLeaks a trove of secret US government documents, many of which were then published around the world.

On March 2, the US military unveiled 22 additional charges against Manning including the serious offense of "aiding the enemy," which carries a potential death sentence. But the army said he would face possible life in prison.

Dozens of activists supporting Manning, including the man who leaked the Pentagon Papers on the Vietnam War, were arrested last month at a military base holding the US soldier in Quantico, Virginia.

Born in Oklahoma in 1987, Manning is an American national but also qualifies as a British citizen by descent through his mother.