WikiLeaks founder Julian Assange has been blocked from seeing evidence in his extradition case, a court has heard.

The 48-year-old appeared at Westminster Magistrates' Court by video-link on Friday for a hearing to extend his custody in Belmarsh Prison, in southeast London.

He is being held in the high-security jail before a full hearing in February when he will fight extradition to the US, where he faces 18 charges, including conspiring to hack into a Pentagon computer.

READ MORE Julian Assange appears confused at extradition hearing, struggles to recall his name

Assange is accused of working with former US Army intelligence analyst Chelsea Manning to leak hundreds of thousands of classified documents.

With unkempt white hair and beard, he appeared uncomfortable as he sat waiting for the hearing to start, clenching his hands together before putting them inside the sleeves of his grey sweater.

He spoke to confirm his name and date of birth, saying: "I do, but I'm an Australian", after the court's legal adviser had mistakenly suggested he was a Swedish national.

AAP

The court heard his lawyers had made a request to the judge, complaining about a lack of access to their client behind bars.

Gareth Peirce, defending Assange, said his legal team was struggling to prepare documents for the case as Assange had no access to the evidence.

"Without Assange's knowledge, some of it is recently acquired evidence, some of it is subject to months of investigation not always in this country, of which he is unaware because of the blockage in visits," she said.

"Despite our best efforts, Assange has not been given what he must be given, and we are doing our utmost to cut through this."

Mr Peirce said the governor of Belmarsh had prioritised family visits over legal visits and asked the judge to step in.

But District Judge Vanessa Baraitser said she had no jurisdiction over the Prison Service.

His appearance comes weeks after more than 60 doctors warned in an open letter addressed to Home Secretary Priti Patel that he could die in prison without urgent medical care.

The medics, from the UK, Australia, Europe and Sri Lanka, expressed "serious concerns" about Assange's fitness to stand trial.

He was jailed for 50 weeks in May for breaching his bail conditions after going into hiding in the Ecuadorian embassy in London to avoid extradition to Sweden over sex offence allegations, which he has always denied.

In November, WikiLeaks welcomed a decision by the Swedish authorities to drop a rape investigation.

Assange has been in custody since he was dramatically removed from the embassy building in April.

He will next appear in court by video-link on 19 December.