US state department spokesman PJ Crowley has tendered his resignation after he called the treatment of Bradley Manning, the solider accused of leaking secret cables to Wikileaks, “stupid.”

Private Manning, whose Facebook profile lists him as being gay, is being held in solitary confinement on suicide watch at the Quantico marine base in Virginia. He is kept shackled at all times.

Mr Crowley made the remarks at an event held at MIT- the Massachusetts Institute of Technology.

When asked about Wikileaks and the “torturing [of] a prisoner in a military brig,” he said: “I spent 26 years in the air force.

“What is happening to Manning is ridiculous, counterproductive and stupid, and I don’t know why the DoD [Department of Defense] is doing it. Nevertheless, Manning is in the right place.”

Mr Crowley resigned after the comments were made public in a blog by the BBC’s Philippa Thomas who was in the audience. Secretary of State Hillary Clinton said she accepted Mr Crowley’s resignation “with regret”. She said that he had always been “motivated by a deep devotion to public policy and public diplomacy”.

A report by Amnesty International said that Private Manning has been held for “for 23 hours a day in a sparsely furnished solitary cell and deprived of a pillow, sheets, and personal possessions since July 2010″.

He is forced to undress on a daily basis and is said to be in an almost permanent state of undress in order to prevent him from committing suicide.