SYDNEY - A cross-party delegation of Australian lawmakers will travel to the United States in September to seek the release of WikiLeaks founder Julian Assange, said his support group on Tuesday, ahead of a visit by Prime Minister Anthony Albanese in October.

The group will urge US officials to drop their attempts to extradite Assange from a British prison to the US, where he is wanted on charges over WikiLeaks’ release of confidential US military records and diplomatic cables.

The delegation will include former deputy prime minister Barnaby Joyce and lawmakers from the Labor government, opposition and Greens party.

Mr Gabriel Shipton, Assange's brother and chair of the Assange Campaign organisation, said the vast majority of Australians did not understand why the US continued to demand his extradition.

“Australians see the US as our closest ally... but right now, Julian is being held hostage by a vengeful US administration, and it’s damaging US-Australian relations,” he said in a statement.

The Australian delegation will meet members of the US Congress and Senate, officials at the State and Justice departments, and think-tanks including the American Civil Liberties Union and Reporters Without Borders.

Assange’s supporters say he has been victimised because he exposed US wrongdoing, including in conflicts in Afghanistan and Iraq. Washington says the release of the secret documents put lives in danger.

Mr Albanese in May said he was frustrated that a diplomatic solution had not been reached to end Assange’s detention, and that he remained concerned about Assange’s mental health.

Support for Assange among US policymakers remains low. Only a few members of Congress have come forward in support of the demand to drop charges against him.

If extradited, he faces a sentence of up to 175 years in a maximum-security prison. REUTERS