Supporters of US soldier Bradley Manning say his treatment in jail amounts to torture, and are calling on the United Nations to investigate.
The US military is accusing the army private of giving classified documents to WikiLeaks, the whistleblower website.
He has been held in solitary confinement at a Marine Corps base in Quantico, Virginia, since his arrest in May, and faces a court martial sometime in 2011.
The office of Manfred Nowak, the UN special rapporteur on torture in Geneva, is to look into the complaints.
Manning was charged in July with leaking classified material, including a video posted by WikiLeaks of a 2007 US Apache helicopter attack in Baghdad that killed a Reuters news photographer and his driver.
Manning has not commented publicly on whether he is the source of the leaks.