WikiLeaks founder Julian Assange left a US court in Saipan where he was freed in a landmark plea deal on Wednesday, but did not address reporters.

The 52-year-old was seen leaving the court in the Northern Mariana Islands, a Pacific US territory, by car. He will now fly to Canberra in his native Australia, WikiLeaks said.

His lawyer Jen Robinson hailed Wednesday as a "historic day" after Assange was freed by the US court.

"Today is a historic day. It brings to an end 14 years of legal battles," she told reporters outside the court in Saipan. "It also brings to an end a case which has been recognised as the greatest threat to the First Amendment in the 21st century."

The WikiLeaks founder has long been wanted by Washington for revealing military secrets.

During the three-hour hearing, Assange pleaded guilty to one criminal count of conspiring to obtain and disclose classified national defence documents but said he had believed the US Constitution's First Amendment, which protects free speech, shielded his activities.

"Working as a journalist I encouraged my source to provide information that was said to be classified in order to publish that information," he told the court.

"I believed the First Amendment protected that activity but I accept that it was ... a violation of the espionage statute."

Chief US District Judge Ramona V Manglona accepted his guilty plea and released him due to time already served in a British jail.

"With this pronouncement, it appears that you will be able to walk out of this courtroom a free man," she said. (AFP/Reuters)