US military prosecutors demanded Monday that Private Bradley Manning spend at least 60 years in jail for handing a vast trove of classified government files to anti-secrecy site WikiLeaks.

Captain Joe Morrow urged the trial judge to impose a tough six-decade sentence and a $100,000 fine to "send a message to any soldier contemplating stealing classified information."

Manning's defense counsel, David Coombs, insisted that this would be far too harsh a sentence for a young man with a chance of rehabilitation, who had expressed remorse and who had cooperated with the court.

Coombs said that if a defendant like Manning had been jailed in 1953, he would have missed a huge slice of modern history and events like the moon landing through the Watergate scandal and the invention of the cellphone to the present day.

He pleaded for a sentence that would allow Manning one day to walk free, find love, get married and live his life, arguing that he had acted out of a humane but naive desire to halt the wars in Iraq and Afghanistan.

"He is a young man, he is a very intelligent man," Coombs said. "He's caring, he's respectful, he's a young man. He was in fact young, was in fact naive, but certainly was good intentioned."

The military judge overseeing Manning's court martial, Colonel Denise Lind, brought the sentencing hearing to a close and said she would briefly reconvene the court on Tuesday before retiring to consider the punishment.

Last month, the 25-year-old former army intelligence analyst was convicted on a raft of espionage and theft charges that could see him jailed for up to 90 years.

Lind is to decide how many years Manning will serve on each charge after a sentencing hearing at Fort Meade, a military base just outside Washington.

Manning pleaded guilty to charges that could see him serve 20 years in a military prison, and Lind has deemed him guilty of several more.

Morrow dismissed the argument that Manning was a naive and troubled soldier who believed he was doing good by exposing abuses in America's conduct of the wars in Iraq and Afghanistan.

Instead, the prosecutor branded the leaks "destructive" and said Manning was a "determined insider who exploited an imperfect system."

Manning, a former intelligence analyst who obtained the files when he was deployed in Iraq, has become a hero to his supporters, who see him as a whistleblower who lifted the lid on America's foreign policy.

More than 100,000 people have signed a petition calling for his nomination for a Nobel Peace Prize.

But the US government painted him as a reckless traitor who put fellow soldiers and his country in danger when he handed 700,000 documents to WikiLeaks, which published them.

The defense contends that Manning hoped to spark a debate on US policy, and that his superiors ignored repeated signs of his emotional distress and should never have sent him to Iraq nor given him security clearance.

Expert witnesses testified during the sentencing hearing that Manning was confused about his gender and sexuality and under enormous psychological stress.

For his part, Manning took the opportunity of the sentencing hearing to apologize for his actions.

"I'm sorry that my actions have hurt people and have hurt the United States," he told Lind last week.

"I want to go forward," he said. "I understand I must pay the price."