WikiLeaks founder Julian Assange has been denied bail after arguing that his release from a UK prison would mitigate his "high risk" of catching coronavirus.

The Australian made the application in the Westminster Magistrates Court on Wednesday, with less than 15 people in attendance due to the coronavirus lockdown.

District Judge Vanessa Baraitser ruled that Assange had absconded before and said that Belmarsh prison is following government guidelines to protect detainees with no confirmed virus cases there yet.

She accepted that government advice may change rapidly but for the time being she denied strict bail for the 48-year-old.

"As matters stand today this global pandemic does not, of itself, yet provide grounds for Mr Assange's release," Judge Baraitser ruled.

"In my view there are substantial grounds to believe that if released today he would not return to face his extradition hearing.

"There are no conditions that allay this concern and this application is therefore refused."

Defence lawyer Edward Fitzgerald QC wore a face mask and his colleague Mark Summers attended via Zoom, while US government lawyers dialled in.

Mr Fitzgerald said Assange has prior chest and tooth infections, and osteoporosis, placing him at a higher risk from the virus.

The QC described prisons as "epidemiological pumps" where diseases spread rapidly and said the defence team had recently been denied entry to Belmarsh because 100 prison staff were self-isolating.

"If he continues to be detained in prison ... there is a real risk that his health and his life will be seriously endangered in circumstances from which he cannot escape," Mr Fitzgerald told the court.

The lawyer also raised the prospect of Assange's next extradition hearing on May 18 being postponed due to lockdown measures.

He said witnesses overseas could be unable to travel and Assange may be unable to meet his counsel in the locked-down prison.

Clair Dobbin for the US government, said Assange's history - including him seeking asylum in the Ecuadorian embassy for almost seven years - showed the high risk he would abscond.

"There are insurmountable hurdles for Mr Assange being granted bail," she said.

"He has been tested before and failed."

But Mr Fitzgerald insisted that Assange's past actions should not be a basis for refusing bail.

"The focus of someone in his vulnerable position, with his family ties here, is on survival, not on absconsion," he said.

The defence also revealed that Assange had a partner and at least two children living in the UK.

Last month, Assange's bid to leave the dock and sit with his lawyers in court was also knocked back by Judge Baraitser.

The US government is trying to extradite Assange to face 17 charges of violating the Espionage Act and one of conspiring to commit computer intrusion over the leaking and publishing of thousands of classified US diplomatic and military files.

Some of those files revealed alleged US war crimes in Iraq and Afghanistan.

The charges carry a total of 175 years' imprisonment.

His extradition case was adjourned until April 7.