A US military judge has reduced the potential prison time for soldier Bradley Manning to 90 years from 136 years by ruling that some sentences for leaking secret files to WikiLeaks should be merged.

Court-martial Judge Colonel Denise Lind convicted Manning last week on 19 criminal counts, including espionage, for providing more than 700,000 secret files to WikiLeaks in the largest unauthorised release of secret data in US history.

In response to defence attorneys' objections that the prosecution was "overreaching" in seeking separate sentences for all the espionage charges, she found that some counts resulted from the same offenses and should be merged to avoid "an unreasonable multiplication of charges".

'Aiding the enemy'

Manning, 25, faces counts including computer fraud and theft charges, but the most serious is aiding the enemy, which carries a possible life sentence.

Manning's supporters also worry a conviction on the most serious charge would serve as a deterrent for other leakers.

The sentencing phase began last week and is expected to last at least until Friday.

Manning's lawyers have portrayed him as naive, but well-intentioned. They argue the soldier's aim was to provoke a broader debate on US military policy, not to harm anyone.

Manning has admitted to sending more than 470,000 Iraq and Afghanistan battlefield reports, 250,000 State Department diplomatic cables and other material, including several battlefield video clips, to WikiLeaks while in Iraq in early 2010.

WikiLeaks published most of the material online.

Manning said he sent the material to expose war crimes and deceitful diplomacy. He claimed that he selected material that would not harm troops or national security.

Prosecutors said al-Qaeda leader Osama bin Laden obtained copies of some of the documents WikiLeaks published before he was killed by US Navy Seals in 2011.