WikiLeaks said Wednesday its founder Julian Assange could travel to the US to face investigation after one of the site's main sources was given clemency -- but only if his rights were "guaranteed".

"Assange is still happy to come to the US provided all his rights are guaranteed," WikiLeaks said on Twitter, the day after US President Barack Obama commuted a prison sentence for former soldier Chelsea Manning.

Manning was sentenced by a military court martial to 35 years behind bars in 2013 for handing 700,000 sensitive military and diplomatic documents to WikiLeaks.

Assange described Manning as "a hero, whose bravery should have been applauded not condemned".

The WikiLeaks founder said last week he would abandon his refuge in the Ecuadoran embassy in London, where he has been since 2012, if the United States agreed to free Manning.

One of the Australian's lawyers, Barry Pollack, said Obama's decision was "well short" of his client's earlier request.

"Mr. Assange welcomes the announcement that Ms. Manning's sentence will be reduced and she will be released in May, but this is well short of what he sought," Pollack told AFP.

"Mr. Assange had called for Chelsea Manning to receive clemency and be released immediately," he added.

Washington has maintained the threat of prosecuting Assange over the 2010 documents leak, although no charges have been publicly filed.

Pollack said he has repeatedly asked the US Department of Justice to clarify his client's status and there has not been a public extradition request.

"I remain willing to discuss Mr. Assange's situation with the DOJ and call for it to announce that it will not pursue any charges against Mr. Assange," he said.

Assange has been living in the Ecuadoran embassy to avoid extradition to Sweden to face sexual assault allegations, which he says are politically motivated and intended as a stepping stone to extraditing him to the US.

- Manning release 'overdue': Amnesty -

Amnesty International campaigned for Manning's release and on Tuesday the human rights organisation described Obama's decision to commute her sentence as "long overdue".

"It is unconscionable that she languished in prison for years while those allegedly implicated by the information she revealed still haven't been brought to justice," Margaret Huang, executive director of Amnesty International USA, said in a statement.

The transgender soldier was tried as Bradley Manning and she has since been held in an all-male prison, where she has twice attempted suicide.

Manning has become a symbol for WikiLeaks, along with Edward Snowden, a former contractor of the US National Security Agency who fled to Russia after revealing a highly classified global communications and internet surveillance system.

Snowden was not included in the outgoing president's list of commutations or pardons, but he posted a "Thanks, Obama" message on Twitter regarding Manning's release.