The 47-year-old is poised to move out of the medical ward of London's Belmarsh prison and on Tuesday shared a moving hug with father John Shipton, who he has not seen since Christmas.

Shipton was joined by Chinese artist and activist Ai Weiwei for the visit at the high security jail.

Assange was moved to a medical ward last month amid grave concerns about his health.

But Shipton told reporters outside Belmarsh that his son's weight has "stabilised", and that he expects to be moved back in the prison population in the next day or so.

Assange is serving a 50-week prison sentence after being dragged out of the Ecuadorian embassy in April and jailed for a bail violation.

"It was just very moving to see Julian, particularly in those circumstances, coming out of sick bay and having lost 10kg weight," said Shipton, 74.

He said he shared a hug with his son which he described as "not a hug that you want to end".

Shipton said Assange asked him to move from Melbourne to the UK and he replied "yes, I'll be here in August".

"His movements have become fine and delicate, and he thinks through things carefully before he says them, so he doesn't speak quickly like I am now," Shipton said, adding that his son had not lost his spirit.

"The spark is there. The fight is there. Just the fire's a little banked."

An emotion Shipton described his son as "a gentle intellectual" and said he was was moved to the sick bay due to weight loss.

Ai Weiwei said Assange, who was recently visited at Belmarsh by Pamela Anderson, is "under psychological stress and pressure", adding that he was "very happy" to see his visitors.

Asked if he thinks Assange remains optimistic about his future, the artist said: "No I don't think so. I think he's deeply worried."

An investigation has been reopened into an allegation of rape in Sweden, which Assange has always denied.

He also faces an extradition request from the US on allegations of spying.

The US Justice Department indicted Assange on 18 counts. He is accused of working with former US army intelligence analyst Chelsea Manning in "unlawfully obtaining and disclosing classified documents related to the national defence", a statement said.

Australian Associated Press