US President Barack Obama commuted the prison sentence of Chelsea Manning on Tuesday, allowing the Army intelligence officer who leaked scores of classified documents to go free nearly three decades early.

Manning, who will leave prison in May, was one of 209 inmates whose sentences Obama was shortening, a list that includes Puerto Rican nationalist Oscar Lopez-Rivera. Obama also pardoned 64 people, including retired Gen. James Cartwright, who was charged with making false statements during a probe into disclosure of classified information.

"These 273 individuals learned that our nation is a forgiving nation," said White House counsel Neil Eggleston, "where hard work and a commitment to rehabilitation can lead to a second chance, and where wrongs from the past will not deprive an individual of the opportunity to move forward."

The actions are permanent, and can't be undone by President-elect Donald Trump. White House officials said Obama would grant clemency to more individuals on Thursday - his final day in office - but that batch was not expected to include prominent individuals like Manning.

A former Army intelligence analyst, Manning has been serving a 35-year sentence for leaking more than 700,000 classified government and military documents to WikiLeaks, along with some battlefield video. She was convicted in military court in 2013 of six violations of the Espionage Act and 14 other offenses and has spent more than six years behind bars. She asked Obama last November to commute her sentence to time served.

Known as Bradley Manning at the time of her 2010 arrest, Manning came out as transgender after being sentenced, and LGBT rights groups took up her cause and lobbied the president to grant her clemency. She was held at a men's prison in Fort Leavenworth, Kansas, and filed a transgender prisoner rights lawsuit, although the military did approve gender-reassignment hormone therapy.

She attempted suicide twice last year, according to her lawyers, citing her treatment in prison. Manning has acknowledged leaking the documents, but has she did it to raise public awareness about the effects of war on civilians.

White House officials said the president was inclined to grant clemency to Manning because she had expressed remorse for her crimes and had served several years of her sentence. The officials briefed reporters on a conference call on condition of anonymity.

"We are all better off knowing that Chelsea Manning will walk out of prison a free woman, dedicated to making the world a better place and fighting for justice for so many," said Chase Strangio, an American Civil Liberties Union attorney representing Manning, adding that Obama's action could "quite literally save Chelsea's life."

But Obama's move was panned by Manning's critics, including several prominent Republicans. House Speaker Paul Ryan called the move "just outrageous".

"Chelsea Manning's treachery put American lives at risk and exposed some of our nation's most sensitive secrets," said Ryan. "President Obama now leaves in place a dangerous precedent that those who compromise our national security won't be held accountable for their crimes."

Manning's commutation came as Obama, days before leaving office, wielded his clemency powers with newfound vigor. Though most of the beneficiaries were low-level drug offenders or little-known financial criminals, a few prominent names made the list, too.

Cartwright, the former vice-chairman of the Joint Chiefs of Staff who received a pardon, had pleaded guilty in October to making false statements during an investigation into a leak of classified information about a covert cyberattack on Iran's nuclear facilities.

Commutations reduce sentences being served but don't erase convictions. Pardons generally restore civil rights, such as voting.