The German government on Wednesday expressed unease over extradition proceedings against Julian Assange, urging authorities to consider the WikiLeaks founder's physical and mental health.

"I am following with concern the extradition proceedings in the UK against WikiLeaks founder Julian Assange," the government's human rights commissioner Baerbel Kofler said in a statement.

Assange, 49, is being held in the high-security Belmarsh jail in London awaiting a January 4 decision by a British judge on a United States extradition request, in a case seen by his supporters as a cause celebre for media freedom.

The Australian publisher faces 18 charges in the US relating to the 2010 release by WikiLeaks of 500,000 secret files detailing aspects of military campaigns in Afghanistan and Iraq, including potential war crimes.

"The human rights and humanitarian aspects of a possible extradition must not be overlooked," Kofler said. 

"It is imperative that the physical and mental health of Julian Assange be taken into account when deciding whether to extradite him to the US," she added, stressing Britain is "bound by the European Convention on Human Rights".

First arrested on December 7, 2010, Assange could be jailed for up to 175 years if convicted.

In 2012, while on bail, he dodged an extradition attempt by Sweden by claiming political asylum in Ecuador's embassy in London.

He remained in the small diplomatic mission for seven years and the Swedish charges were later dropped.

But he was turned over to British police in April 2019 after a change of government in Quito, then jailed for having skipped bail.

Washington then served a formal extradition request.