The office of Manfred Nowak, special rapporteur on torture in Geneva, received a complaint from one of Pfc. Bradley Manning's supporters alleging conditions in a Marine Corps brig in Quantico, Va., amount to torture, said spokesperson Xabier Celaya. Visitors say he spends at least 23 hours a day alone in a cell.
The U.N. could ask the United States to stop any violations it finds.
The Pentagon has denied mistreating Manning. A Marine Corps spokesman says the military is keeping Manning safe, secure and ready for trial.
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Manning was charged in July with leaking classified material, including video posted by WikiLeaks of a 2007 U.S. Apache helicopter attack in Baghdad that killed a Reuters news photographer and his driver. He is suspected of leaking troves of other material to the government secret-spilling site, which is in the process of posting more than 250,000 secret U.S. State Department cables.
Manning has not commented publicly on whether he is the source of the leaks. WikiLeaks founder Julian Assange said the organization's "technology is set up so we don't know" the sources of the material it gets.
Nowak is the special investigator on torture, working for the U.N. Human Rights Council. Rapporteurs regularly assess complaints from alleged victims of human rights violations. If a complaint is verified as legitimate, the investigator sends an urgent letter or appeal to the government that it believes has committed the violation.
In an interview with MSNBC on Wednesday, Assange called Manning a political prisoner and said he believes the U.S. is trying to get the soldier to testify against him. He called on human-rights organizations to investigate.
"If we are to believe the allegations, then this man acted for political reasons. He is a political prisoner in the United States. He has not gone to trial. He has been a political prisoner without trial in the United States for some six or seven months," Assange said.
"His conditions have been getting worse and worse and worse in his cell as they attempt to pressure him into testifying against me. That's a serious problem."
Assange has not been charged in connection with leaked documents but was jailed in England this month after two women in Sweden accused him of sex crimes, including rape. He was freed on bail last week and confined to a supporter's country estate while he fights extradition to Sweden, where authorities want to question him.
Assange said it would be "absolute nonsense" for the U.S. to try to make Manning a witness in a conspiracy case against him. "I never heard of the name Bradley Manning before it appeared in the media," he said.