US President Barack Obama on Tuesday slashed the sentence of transgender army private Chelsea Manning, who had been sentenced to 35 years behind bars for handing classified US documents to WikiLeaks.

Obama pardoned 64 people and commuted the sentences of 209 others -- including 29-year-old Manning, who will now be released in May -- in one of his final acts as president.

Manning was convicted in August 2013 of espionage and other offenses, after admitting to the leak of 700,000 sensitive military and diplomatic documents.

The cache included military logs from wars in Iraq and Afghanistan and cables offering sensitive -- and often embarrassingly bareknuckle -- diplomatic assessments of foreign leaders and world events.

Then Bradley Manning pleaded guilty and was sentenced by military court martial. She has since been held in an all-male prison, at times in solitary confinement, and has attempted to commit suicide twice.

Activists had argued her sentence is excessive and point to the psychological frailty of the transgender soldier.

"This move could quite literally save Chelsea's life," said Chase Strangio of the American Civil Liberties Union.

Still, Obama's move is something of a surprise, coming in the midst of a scandal over election-related hacking.

Obama has imposed sanctions on Russian intelligence services over the hack of Democratic party emails, some of which were released via WikiLeaks.

- 'Victory' -

In recent weeks the White House had refused to be drawn on a possible commutation or pardon.

But spokesman Josh Earnest did attempt to paint a stark difference between Manning -- who went through the courts and admitted wrongdoing -- and the likes of Edward Snowden.

Snowden, a former NSA contractor, fled to Hong Kong and then Russia in 2013 after revealing a highly classified global communications and internet surveillance system.

He was not on Obama's list of commutations or pardons, but did tweet his thanks.

"Let it be said here in earnest, with good heart: Thanks, Obama."

WikiLeaks -- which has been linked to last year's election hacks -- claimed "victory" and thanked those who campaigned on Manning's behalf.

"Your courage & determination made the impossible possible," the group tweeted, citing founder Julian Assange.

But there was no suggestion Assange -- who is holed up in the Ecuadoran embassy in London -- would make good on a promise to be extradited to the United States if Manning was freed.

"If Obama grants Manning clemency Assange will agree to US extradition despite clear unconstitutionality of DoJ case," the group tweeted last week.

White House officials dismissed any link between WikiLeak's pledge and Obama's decision on Manning.

- Backlash -

Republicans expressed outrage at Obama's decision.

"This is just outrageous," said House Speaker Paul Ryan. "Chelsea Manning's treachery put American lives at risk and exposed some of our nation's most sensitive secrets."

"President Obama now leaves in place a dangerous precedent that those who compromise our national security won't be held accountable for their crimes."

Republican Senator Tom Cotton, who is tipped as a possible future leader of the party, expressed fury at Obama's decision, saying "we ought not treat a traitor like a martyr."

"I don't understand why the president would feel special compassion for someone who endangered the lives of our troops, diplomats, intelligence officers, and allies."

Republicans have been on the back foot over Russian hacks that appeared to help their presidential candidate Donald Trump.

Among the others who received commutations was Puerto Rican nationalist Oscar Lopez-Rivera, who has been in prison for more than three decades on terrorism charges.

Obama also pardoned James Cartwright, a former four-star general who lied to the FBI about his discussions with journalists about Iran's nuclear program.

Another round of commutations is expected on Thursday, officials suggested.

Many will look to see whether the new list includes Bowe Bergdahl, a US Army sergeant held captive for five years by the Taliban before his release in a prisoner swap. He is due to be court-martialed for desertion.

Other names omitted Tuesday were General David Petraeus -- who pleaded guilty to improperly sharing classified information -- and Obama's ally Hillary Clinton.

There had been wild speculation that Obama may choose to preemptively pardon her, forestalling any Trump-led prosecution over her handling of email as secretary of state.

Presidents can theoretically pardon people before they are even sentenced.

Trump takes office on Friday.