US authorities accuse Australian-born Assange, 50, of 18 counts relating to WikiLeaks' release of vast troves of confidential US military records and diplomatic cables which they said had put lives in danger.

His supporters cast Assange as an anti-establishment hero who has been victimised by the United States for exposing US wrongdoing in Afghanistan and Iraq.

The United States on Friday won an appeal against a ruling by a London District judge that Assange should not be extradited because he would likely commit suicide in a US prison.

"The court allows the appeal," Justice Timothy Holroyde said in the London High Court's decision on Friday.

The judge said he was satisfied with a package of assurances given by the United States about the conditions of Assange's detention.

These included pledges not to hold him in a so-called "ADX" maximum security prison in Colorado and that he would be transferred to Australia to serve his sentence if convicted.

But further hurdles remain before Assange can be sent to the United States and the legal wrangling is likely to go to the Supreme Court, the final court of appeal.

Assange's fiancee Stella Moris said his legal team would appeal the decision.

"How can it be fair, how can it be right, how can it be possible, to extradite Julian to the very country which plotted to kill him?" she said.

"We will appeal this decision at the earliest possible moment."

Justice Holroyde said the case must now be remitted to Westminster Magistrates Court with the direction that judges send it to the British government to decide whether or not Assange should be extradited.

Assange, who denies any wrongdoing, was not in court. He remains in London's high-security Belmarsh prison, where he has been for more than two-and-a-half years.

WikiLeaks came to international prominence when it published a US military video in 2010 showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff. It then released thousands of secret classified files and diplomatic cables.

US prosecutors and security officials regard Assange as a reckless and dangerous enemy of the state whose actions imperilled the lives of agents named in the leaked material.

His admirers have hailed Assange as a hero for exposing what they describe as abuse of power by modern states and for championing free speech. A British court has opened the door for WikiLeaks founder Julian Assange to be extradited to the United States.

Australian Associated Press