Wikileaks founder Julian Assange has pleaded guilty in a US court under a deal allowing him to walk free following a 14-year legal battle.

He entered the formal plea to a single charge in the Northern Mariana Islands, an American territory in the Pacific, two days after leaving a British prison.

In return, he was sentenced to time already served and released to fly to his native Australia and be reunited with his family.

US officials were pursuing the 52-year-old over a huge disclosure of secret files in 2010, which they said put lives in danger.

Assange arrived at court on Wednesday morning local time in Saipan, the capital of the Northern Marian Islands, alongside a team that included Australia's ambassador to the US Kevin Rudd.

At the hearing, Assange admitted a felony charge of conspiracy to obtain and disclose national defence information.

He told the court that when he published the classified files leaked to him in 2010, he was a journalist and believed he would be protected by the First Amendment of the US Constitution, which covers freedom of the press.

After the sentencing, Assange's lawyer, Barry Pollack, said: "Wikileaks's work will continue and Mr Assange, I have no doubt, will be a continuing force for freedom of speech and transparency in government.

"He is a powerful voice and a voice that cannot and should not be silenced."

The hearing took place on the remote Pacific archipelago as the US commonwealth is much closer to Assange's native Australia than US federal courts in Hawaii or the continental US.

Assange spent the last five years behind bars at London's high-security Belmarsh Prison, fighting US attempts to extradite him.

In 2010, he faced separate charges of rape and sexual assault in Sweden, which he denied. He spent seven years hiding in Ecuador's London embassy, claiming the Swedish case would lead him to be sent to the US.

Swedish authorities dropped that case in 2019 and said that too much time had passed since the original complaint, but UK authorities later took him into custody. He was tried for not surrendering to the courts to be extradited to Sweden.

In the US, Assange was charged with conspiracy to obtain and disclose national defence information, following the massive Wikileaks disclosure in 2010.

Wikileaks had released a video from a US military helicopter which showed civilians being killed in the Iraqi capital Baghdad.

It also published thousands of confidential documents suggesting that the US military had killed hundreds of civilians in unreported incidents during the war in Afghanistan.

The revelations became a huge story, prompting reaction from all corners of the globe, and led to intense scrutiny of American involvement in foreign conflicts.