US President Barack Obama firmly defended his decision to cut nearly three decades off convicted leaker Chelsea Manning's prison term at his final White House press conference.

Obama said that he granted clemency to Manning because she had gone to trial, taken responsibility for her crime and received a sentence that was harsher than other leakers have received.

He emphasized that he had merely commuted her sentence, not granted a pardon, which would have symbolically forgiven her for the crime.

"I feel very comfortable that justice has been served," Obama said on Wednesday.

Manning was convicted in 2013 of violating the Espionage Act and other crimes for leaking more than 700,000 classified documents while working as an intelligence analyst in Baghdad.

Formerly known as Bradley Manning, she declared as transgender after being sentenced to 35 years in prison and had served more than six years before Obama commuted her sentence on Tuesday, with a release date set for May.

"The notion that the average person who was thinking about disclosing vital, classified information would think that it goes unpunished, I don't think would get that impression from the sentence that Chelsea Manning has served," Obama said.

Obama said he saw no contradiction in granting clemency to Manning even as he warns about Russia's hacking of the US presidential campaign, in which stolen emails were released publicly by WikiLeaks.

He said he wasn't motivated by WikiLeaks' recent pledge on Twitter that founder Julian Assange would agree to extradition to the US if Obama commuted Manning's sentence.

"I don't pay much attention to Mr. Assange's tweets, so that wasn't a consideration," Obama said.

Although Obama had long intended to take one last round of questions before leaving office, White House officials said that in recent days, Obama became intent on using the occasion to draw a symbolic contrast with Trump on issues of accountability and press freedoms.

Trump's team has said it's considering changes to the traditional daily press briefing and to the location of news conferences, stoking concerns among journalists that their ability to cover the presidency may be scaled back.