Australia must help defeat Islamic State jihadists, but should not revoke the citizenship of terrorism supporters in Iraq, lawyer Geoffrey Robertson says.

Australia has a moral obligation to help oust Islamic State (IS) in Iraq and Syria, international law expert Geoffrey Robertson says.

The Abbott government is waiting for Washington to make a specific request for help before committing Australian troops to a US-led effort to train Iraqi and Kurdish forces fighting the jihadist group.

Mr Robertson, a QC who has served as president of the UN war crimes court in Sierra Leone, said Australia had an obligation to stop a group that executed people without trial.

“That sets up a moral, imperative duty to intervene to protect innocent civilians,” he told AAP in Brisbane on the sidelines of Griffith University’s Global Integrity Summit.

“With Islamic State, there must be an element of force in order to remove them, remove their threat.”

Mr Robertson said the term terrorist was not “nearly strong enough” to describe the group that beheaded American journalists James Foley and Steven Sotloff.

“I would think barbarians for a start, war criminals, those who commit crimes against humanity, even genocidaires, to use the French term, because they are out to kill for racial or religious reasons,” he said.

But the Australian expatriate barrister, professor and author, who is based in London, said the federal government should refrain from revoking the citizenship of those who travelled to the Middle East to support IS.

“There are a 100 or so Australians who are in this position. I don’t think they should be rendered stateless by not being allowed to return by having their passports taken away,” he said.

“That would be a breach of international law.”

Australia’s criminal code was amended in 1995 so Australians could be jailed for 25 years or given life for taking part in crimes against humanity and genocide.

“We should set up a special prosecutor to prosecute them for those offences when or if they return,” he said.