U.S. soldier Bradley Manning on Wednesday told a military court "I'm sorry" for giving war logs and diplomatic secrets to the WikiLeaks website three years ago, the biggest breach of classified data in the nation's history.

"I am sorry that my actions hurt people. I'm sorry that they hurt the United States," the 25-year-old U.S. Army Private First Class told the sentencing phase of his court-martial. "I am sorry for the unintended consequences of my actions ... The last few years have been a learning experience."

Manning spoke quietly and non-defiantly in his first extensive public comments since February.

Manning faces up to 90 years in prison for providing more than 700,000 documents, battle videos and diplomatic cables to WikiLeaks, hurling the pro-transparency website and its founder, Julian Assange, into the world spotlight.

Defense lawyers seeking a milder sentence rested their case on Wednesday after Manning's statement. With about a dozen witnesses including Army superiors, mental health professionals and Manning's own sister, they sought to show Judge Colonel Denise Lind that commanders ignored signs of mental stress.

An Army psychologist testified at the hearing at Fort Meade, Maryland, that Manning, who is gay, felt isolated because he was wrestling with his gender identity. Another mental health specialist testified that Manning had hoped to end war.

"I should have worked more aggressively inside the system ... Unfortunately, I can't go back and change things," Manning, wearing his dress uniform and glasses, his hair in a crew cut, said from the witness stand.

He did not appear to be reading from notes and looked at the judge and around the room as he spoke.

"I understand I must pay a price for my decisions," Manning continued in his first lengthy public statement since February.

"I want to be a better person, to go to college, to get a degree and to have a meaningful relationship with my sister and her family."

Manning was convicted of 20 charges, including espionage and theft, on July 30. He was found not guilty of the most serious count, aiding the enemy, which carried a life sentence.

A military spokesman said the judge would most likely sentence Manning next week at the earliest.

Prosecutors, who have argued that Manning was an arrogant soldier who aided al Qaeda militants and harmed the United States with the release of the documents, will have an opportunity to rebut the defense case on Friday.

'Hyper-masculine environment'

Captain Michael Worsley, who treated Manning from December 2009 to May 2010 during his deployment in Iraq, testified that the stress Manning had felt from his job as a low-level intelligence analyst was compounded by being in a "hyper masculine environment" of a combat zone.

"Being in the military and having a gender identity issue does not exactly go hand in hand," Worsley said. "You put him in that kind of environment, this hyper masculine environment ... with no coping skills, the pressure would have been incredible."

That pressure reached a peak when Manning punched another soldier, Worsley said. He said he had met infrequently with Manning and had no input from his superiors until the punching incident.

Navy Reserve Captain David Moulton, a forensic psychiatrist, said he had diagnosed Manning as having gender dysphoria, or wanting to be the opposite sex, as well as narcissism and obsessive-compulsive behavior. Those problems were compounded by the unraveling of a romantic relationship, the stress of serving in a Baghdad combat base and post-adolescent idealism, he said.

"Manning was under the impression that the information he was giving was going to change the way the world saw the wars in Iraq and Afghanistan, and future wars, actually," said Moulton, who interviewed Manning for about 21 hours and spent more than 100 hours on the case.

'No war was worth it'

The psychiatrist said Manning thought it "would lead to a greater good. Society as a whole would come to the conclusion that the war wasn't worth it, that no war was worth it."

Manning's sister, Casey Major, 36, of Oklahoma City, testified about how her parents' alcoholism and infighting compelled her to play a lead role in raising her brother.

She recounted their early days together in Crescent, Oklahoma, reviewed family photos, and said, "I just hope he can be who he wants to be. I just hope he can be happy."

Troubles at home prompted a teenaged Manning to move to Potomac, Maryland, to live with his aunt, said Debra van Alstyne, the aunt who took him in. She recalled that Manning had joined the Army in the hope of going to college on the GI bill, after having difficulty balancing classes and a job at Starbucks.

The material Manning released that shocked many around the world was a 2007 gunsight video of a U.S. Apache helicopter firing at suspected insurgents in Baghdad. A dozen people were killed, including two Reuters news staff. WikiLeaks dubbed the footage "Collateral Murder."

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Manning, described by his superiors as an Internet expert, faces the prospect of decades of monotonous prison life - with no online access - once he is sentenced.

(Additional reporting by Tom Ramstack; Editing by Scott Malone, Grant McCool and Andrew Hay)