TORONTO - The American government has withdrawn a witness against Omar Khadr in an effort to hide evidence of its mistreatment of the Canadian during his detention at Guantanamo Bay, his Pentagon-appointed lawyer says.

The special agent had been slated to testify at Khadr's war-crimes trial next month about a self-incriminating statement the prisoner gave in December 2004.

Khadr's legal team maintains the statement was coerced and wanted to question the U.S. Defence Department agent about the statement.

Lt.-Cmdr. Bill Kuebler, who is defending Khadr before a military commission, said the government is trying to cut off defence probing of his abuse by U.S. authorities.

"It's a shocking concession by the government that effectively (says) the things that Omar relates about his mistreatment in 2003 and 2004 are true, otherwise they wouldn't be seeking to side-step the issue by withdrawing this witness," Kuebler said from Washington.

"It corroborates or confirms that . . . this kid was absolutely traumatized and mistreated by U.S. government authorities and now the U.S. government is trying to continue to cover that up."

Marine Maj. Jeff Groharing, who is prosecuting Khadr, confirmed the withdrawal of the witness but did not offer an explanation.

Khadr was held at the infamous Bagram prison following his capture in Afghanistan in July 2002 at the age of 15, before being shipped to Guantanamo Bay.

In questioning by intelligence agents during his detention, Khadr related details of the firefight that left him badly injured and an American soldier dead.

He made several incriminating statements, which he disavowed in February 2003, when he was finally allowed to talk to Canadian officials.

Khadr, now 22, claims in an affidavit that his recantation sparked reprisals from his captors, including sleep deprivation, being held in stress positions and kept in isolation.

His lawyers argue their client was clearly afraid of what would happen if he didn't say what his interrogators wanted him to say -- hence the incriminating statement in December 2004.

"A key 2004 statement allegedly taken from Mr. Khadr as much as acknowledges that an inculpatory statement had to be dragged out of him," their legal submissions state.

"Mr. Khadr expressed fear of retribution for his initial unwillingness to say what the agents wanted to hear, expressing his hope that his `other interrogator would not be mad at him' for initially failing to confess."

The defence had planned to make Khadr's treatment during his entire captivity from July 2002 until December 2004 an issue in his trial but corroborating documentation related to the earlier time frame has been lost.

"Evidence is out there to suggest that the reason we don't have (earlier) materials . . . is that the government was in the process of systematically destroying such information to prevent its use in legal proceedings," Kuebler said.