WASHINGTON (Reuters) - The U.S. soldier accused of leaking secret documents that appeared on the WikiLeaks website was transferred to a Kansas military jail on Wednesday, the Pentagon said.

Bradley Manning is being held during the investigation of charges involving reams of sensitive diplomatic and military documents he is accused of leaking while posted as an intelligence analyst in Iraq.

He arrived safely at Fort Leavenworth's Joint Regional Correctional Facility on Wednesday afternoon, a Pentagon spokesman said in a statement.

Manning had been in detention at a Marine base in Quantico, Virginia, since May of last year.

"Given the length of time he's been in pretrial confinement at Quantico (and) the likely period of pretrial confinement in the future ... we reached the judgment this would be the right facility for him," Jeh Johnson, the Pentagon's general counsel, said about the move in a Tuesday briefing.

"At the request of Private Manning's defense counsel, an assessment is under way to determine whether Private Manning is mentally competent in this case in the event it goes to trial," Johnson also said.

Manning's lawyers have complained the 23-year-old soldier was mistreated at the Virginia Marine brig where he had been held since May last year.

Kept alone in his cell 23 hours per day, the Pentagon said he had been forced to sleep naked and woken repeatedly during the night to ensure he is safe. The Pentagon said this had happened only a few times.

Last month, President Barack Obama said he had been assured by the Pentagon that Manning's treatment was appropriate.

Manning will be placed in a single cell in Ft. Leavenworth, and he would not receive abusive treatment, officials said.

The WikiLeaks publication of the documents Manning is suspected of leaking was a blow to U.S. diplomacy as allies and adversaries saw themselves mocked or second-guessed in secret diplomatic cables.

U.S. military officials have not said when Manning's trial might begin. "We are probably months off from a trial," Johnson said.

(Reporting by Jerry Norton and Missy Ryan; Editing by Greg McCune)