In one of his final acts before leaving office, President Barack Obama on Tuesday shortened the prison sentence of former U.S. military intelligence analyst Chelsea Manning, who was responsible for the biggest breach of classified materials in U.S. history to anti-secrecy group WikiLeaks in 2010.

A White House official said there was no connection between Manning's commutation and renewed U.S. government concern about WikiLeaks in last year's presidential election, or a promise by its founder Julian Assange to accept extradition if Manning was freed.

Manning has been a focus of a worldwide debate on government secrecy since she provided more than 700,000 documents, videos, diplomatic cables and battlefield accounts to WikiLeaks - a leak for which she was sentenced to serve 35 years in prison and now reduced to seven years.

Wikileaks also published emails in the weeks leading up to the Nov. 8 presidential election that U.S. intelligence agencies have concluded that Russian intelligence agencies hacked from the Democratic National Committee and the accounts of leading Democrats, part of a campaign to influence the election.

A WikiLeaks tweet on Tuesday quoted Assange as saying: "Thank you to everyone who campaigned for Chelsea Manning's clemency. Your courage & determination made the impossible possible."

Assange has been holed up at Ecuador's London embassy since 2012 to avoid extradition to Sweden for the investigation of allegations, which he denies, that he committed rape in 2010. He has said he fears extradition from Sweden to the United States, where there is an open criminal investigation into the activities of WikiLeaks.

"The president's decision to grant clemency and offer commutation to Chelsea Manning was not influenced in any way by public comments from Assange or the WikiLeaks organization," a White House official said on a conference call with reporters.

Manning, formerly known as U.S. Army Private First Class Bradley Manning, was born male but revealed after being convicted of espionage that she identifies as a woman. The White House said her sentence would end on May 17 this year.

Manning, who twice tried to kill herself last year and has struggled to cope as a transgender woman in the Fort Leavenworth, Kansas, men's military prison, accepted responsibility for leaking the material.

Manning was working as an intelligence analyst in Baghdad in 2010 when she gave WikiLeaks a trove of diplomatic cables and battlefield accounts that included a 2007 gunsight video of a U.S. Apache helicopter firing at suspected insurgents in Iraq, killing a dozen people including two Reuters news staff.

Her attorney had her sentence exceeded international legal norms.

Manning's clemency was criticized by Republican Senator Tom Cotton, who said Manning endangered troops, intelligence officers, diplomats and allies with the leaks.

"We ought not treat a traitor like a martyr," Cotton said.

But civil rights groups praised the move, calling it overdue.

"Chelsea Manning exposed serious abuses, and as a result her own human rights have been violated by the U.S. government for years," said Margaret Huang, executive director of Amnesty International USA.

Obama also pardoned retired U.S. Marine Corps general James Cartwright who pleaded guilty in October to making false statements to the FBI during an investigation into leaks of classified information.

Manning was among 209 commutations granted by Obama on Tuesday and Cartwright was among 64 pardons.

Most of the commutations were a part of Obama's effort to reduce the number of people serving long sentences for non-violent drug offences. The White House official, who spoke to reporters on condition of anonymity, said Obama would likely announce more such commutations on Thursday.