Nils Melzer, who monitors torture and other forms of inhuman treatment around the world, issued an open letter to Trump ahead of a British court ruling on January 4 that will determine whether Assange will be extradited from Britain to the United States.

Australian national Assange is being held in the Belmarsh high security prison in London.

He could face up to 175 years in prison in the US, on charges of publishing classified material on military operations in Iraq and Afghanistan, which exposed war crimes by US soldiers.

"In pardoning Mr Assange, Mr President, you would send a clear message of justice, truth and humanity to the American people and to the world," Melzer wrote.

"You would rehabilitate a courageous man who has suffered injustice, persecution and humiliation for more than a decade, simply for telling the truth."

Assange's health has seriously deteriorated behind bars in Britain "to the point where his life is now in danger", Melzer, a high-profile Swiss human rights scholar and former Red Cross official, said.

Australian Associated Press