Two Welsh MEPs have added their names to a growing list of their colleagues concerned about the alleged treatment of Wikileaks suspect Bradley Manning.
The US Army analyst, who grew up in Pembrokeshire, is due in court later this month accused of passing sensitive information to the website.
He has been in military custody in the US since May 2010, accused of making intelligence available to "the enemy".
Plaid Cymru's Jill Evans and Labour's Derek Vaughan have signed the letter.
Ms Evans said US treatment of Pte Manning had become an important human rights issue.
During his detainment at a military prison at Quantico, Virginia, it is alleged that the soldier was held in solitary confinement for 23 hours a day and often made to sleep without clothing or bedding.
"I think anyone who read about the conditions under which he has been held would be very disturbed," said Ms Evans.
"It is totally out of proportion to anything that he has been accused of."
An open letter has now been sent to President Obama signed by more than 60 members of the European Parliament, including Ms Evans and the Welsh Labour MEP Derek Vaughan.
The letter calls on the US president to allow Pte Manning to meet the United Nation's special rapporteur on torture, Juan Méndez.
The signatories also state they are concerned that the soldier has been charged with the offence of 'aiding the enemy', which could carry the death penalty in the US.
Ms Evans said she hoped the letter would carry added weight in America as a trial date approaches.
"Human rights is an issue that the European Parliament has always fought very hard to protect, and something we feel we have a real role in doing on the world stage," she added.
The 23-year-old is from Oklahoma but went to Tasker Milward school in Haverfordwest, where he lived with his mother.
He is accused of passing hundreds of thousands of diplomatic cables to the whistleblowing website, detailing operational information in Iraq and Afghanistan.
After mounting concerns were raised in America about his plight behind bars, US authorities announced in April that the soldier was being transferred to another military prison in Kansas, where the custodial regime was viewed as being more relaxed.
The US government said the decision had been taken because the Quantico facility had not been designed to hold pre-trial prisoners on a long-term basis.
Pte Manning's lawyers say they now expect a pre-trial case, called an Article 32 hearing, to go ahead on 16 December.
Proceedings are expected to last around five days, after which recommendations will be made to a general, who will decide whether to proceed to a full trial.
Pentagon officials have previously said Pte Manning is being held in appropriate conditions considering the seriousness of the charges against him.
He has been charged with using unauthorised software on government computers to download classified information and to make intelligence available to "the enemy", as well as other counts related to leaking intelligence and theft of public records.