Britain's High Court on Wednesday finished hearing two days of arguments over whether to grant WikiLeaks founder Julian Assange a fresh appeal against his extradition to the United States to face espionage charges.

Two senior judges heard evidence from his lawyers and those representing Washington, and opted against making an immediate decision on what is likely Assange's final UK bid to block extradition.

"We will reserve our decision," judge Victoria Sharp said as the latest legal proceedings in the long-running case concluded. It is unclear when she and judge Jeremy Johnson will issue their ruling.

Washington indicted Assange, the Australian founder of WikiLeaks, multiple times between 2018 and 2020 over its publication of hundreds of thousands of secret military and diplomatic files on the US-led wars in Iraq and Afghanistan.

Assange has since waged a half-decade battle against being sent there, but has lost successive rulings in recent years. If he fails again, he could be extradited within weeks.

Lawyers for the US government urged the court to reject his arguments on various grounds.

Clair Dobbin said that Assange had "solicited" the secret US files and, in eventually publishing them "indiscriminately" without redactions, that his actions were "unprecedented" and did not constitute journalism.

"The evidence shows that from the time the appellant started WikiLeaks... he sought to recruit individuals with access to classified information," Dobbin added. "He worked with hackers."

In response, one of Assange's lawyers, Mark Summers, hit out at Dobbin's testimony for failing to address that he was exposing "state-level crimes."

"We heard no answer at all," he said, adding "that is protected conduct" under UK law.

Assange was absent from court for the two-day session, and did not follow the proceedings via video due to illness, his lawyer said. (AFP)