UN rights chief Michelle Bachelet said Saturday that the potential extradition and prosecution of Julian Assange raised concerns for media freedom and could have a "chilling effect" on investigative journalism.

Assange, who has been held in a high-security London prison since 2019, has filed an appeal against his extradition from Britain to the United States.

The Australian is wanted to face trial for allegedly violating the US Espionage Act by publishing classified US military and diplomatic files in 2010, related to the wars in Afghanistan and Iraq.

The 51-year-old could face decades in jail if found guilty, but supporters portray him as a martyr to press freedom after he was taken into British custody following nearly seven years inside Ecuador's embassy in London.

"I am aware of health issues which Mr. Assange has suffered during his time in detention, and remain concerned for his physical and mental well-being," Bachelet said in a statement after meeting with the WikiLeaks founder's wife and lawyers on Thursday.

"The potential extradition and prosecution of Mr. Assange raises concerns relating to media freedom and a possible chilling effect on investigative journalism and on the activities of whistle-blowers.

"In these circumstances, I would like to emphasise the importance of ensuring respect of Mr. Assange's human rights, in particular the right to a fair trial and due process guarantees in this case.

"My office will continue to closely follow Mr. Assange's case."

Bachelet's term as the United Nations high commissioner for human rights finishes on Wednesday, after four years in the post.

The former Chilean president's successor has not yet been appointed.

The US-based Assange Defense Committee coalition fighting to free the former computer hacker said the legal battle over his extradition was heating up on multiple fronts.

"Assange's attorneys stressed the legal and human rights implications of the case, while Stella Assange updated Bachelet on the impact years of confinement have had on Julian's health and family," the statement said.

The Assange case has become a cause celebre for media freedom and his supporters accuse Washington of trying to muzzle reporting of legitimate security concerns.