ADRID (Reuters) - Outgoing Defense Secretary Donald Rumsfeld authorized the mistreatment of detainees at Abu Ghraib prison in Iraq, the prison's former U.S. commander said in an interview on Saturday.

Former U.S. Army Brigadier General Janis Karpinski told Spain's El Pais newspaper she had seen a letter apparently signed by Rumsfeld which allowed civilian contractors to use techniques such as sleep deprivation during interrogation.

Karpinski, who ran the prison until early 2004, said she saw a memorandum signed by Rumsfeld detailing the use of harsh interrogation methods.

"The handwritten signature was above his printed name and in the same handwriting in the margin was written: "Make sure this is accomplished,"" she told Saturday's El Pais.

"The methods consisted of making prisoners stand for long periods, sleep deprivation ... playing music at full volume, having to sit in uncomfortably ... Rumsfeld authorized these specific techniques."

The Geneva Convention says prisoners of war should suffer "no physical or mental torture, nor any other form of coercion" to secure information.

"Prisoners of war who refuse to answer may not be threatened, insulted, or exposed to any unpleasant or disadvantageous treatment of any kind," the document states.

A spokesman for the Pentagon declined to comment on Karpinski's accusations, while U.S. army in Iraq could not immediately be reached for comment.

Karpinski was withdrawn from Iraq in early 2004, shortly after photographs showing American troops abusing detainees at the prison were flashed around the world. She was subsequently removed from active duty and then demoted to the rank of colonel on unrelated charges.

Karpinski insists she knew nothing about the abuse of prisoners until she saw the photos, as interrogation was carried out in a prison wing run by U.S. military intelligence.

Rumsfeld also authorized the army to break the Geneva Conventions by not registering all prisoners, Karpinski said, explaining how she raised the case of one unregistered inmate with an aide to former U.S. commander Lt. Gen. Ricardo Sanchez.

"We received a message from the Pentagon, from the Defense Secretary, ordering us to hold the prisoner without registering him. I now know this happened on various occasions."

Karpinski said last week she was ready to testify against Rumsfeld, if a suit filed by civil rights groups in Germany over Abu Ghraib led to a full investigation.

President Bush announced Rumsfeld's resignation after Democrats wrested power from the Republicans in midterm elections earlier this month, partly due to public criticism over the Iraq war.