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The US soldier accused of leaking a trove of secret government documents later published by the Wikileaks website is to be moved to a military prison in Kansas, officials have said.

Pte First Class Bradley Manning has been held pending court martial at a Marine Corps base in Virginia.

His transfer comes amid international concern over his treatment.

His supporters say he has been confined to a cell for 23 hours a day and forced regularly to undress.

Last week, UN special rapporteur on torture Juan Mendez said US officials had blocked his requests for unmonitored visits to Pte Manning, in part aimed at determining whether he had been mistreated.

Ray McGovern, a member of the Bradley Manning Support Network's Advisory Board, said this was "unprecedented".

"What it is, pure and simple, is an intimidation technique, lest any of Bradley Manning's colleagues, any of the other people in the armed forces, be tempted to do what Bradley Manning did, and that was to expose war crimes," he told the ´óÏó´«Ã½ World Service.

'Best interest'

At a press conference at the Pentagon on Tuesday, defence department general counsel Jeh Johnson said Pte Manning would be moved imminently to a pre-trial jail at Fort Leavenworth, in the Mid-western US state of Kansas.

Mr Johnson and other military officials said the Fort Leavenworth jail - which was opened in January - was better equipped to handle long-term pre-trial stays than the Marine Corps base at Quantico, Virginia.

Among other things, the Fort Leavenworth jail has better mental health support and, should officials permit it, better exercise facilities and more opportunities for interaction with other detainees, officials said.

"We have assessed this is in Pte Manning's best interest to move him at this juncture in the case," Mr Johnson said.

"All things considered, we concluded that going forward this is the best facility for him."

Mr Johnson said the transfer should not be interpreted as a criticism of Pte Manning's treatment at Quantico, though he acknowledged senior defence officials had been involved in the process.

Pte Manning's civilian lawyer David Coombs has said the soldier has been under 24-hour surveillance and has been forced to relinquish his clothing before bedding down for the night, then forced to stand naked at roll call.

Officials have repeatedly denied Pte Manning has been mistreated, although last month a top US state department official, spokesman PJ Crowley, resigned after saying the military's treatment of the Wikileaks suspect was "ridiculous and counterproductive".

Pte Manning, an intelligence analyst who joined the US Army in 2007, is suspected of leaking 720,000 diplomatic and military documents, including a database of military records from the Iraq war, Afghan war records, classified diplomatic cables and other materials.

In the past year, Wikileaks has published troves of documents it titled the Iraq War Logs and the Afghan War Diary, and reams of secret US state department cables spanning five decades.

Pte Manning has been charged with using unauthorised software on government computers to download classified information and to make intelligence available to "the enemy", and other counts related to leaking intelligence and theft of public records.

He is currently undergoing a mental health evaluation aimed at determining his competence to stand trial, officials have said.