At least 70 Australians are said to be fighting with militants in Syria and Iraq

At least 20 Australians have been killed fighting alongside militants in Iraq and Syria, the Australian government has said.

Several of them died around the Syrian border town of Kobane, Attorney General George Brandis.

Mr Brandis said Islamic State (IS) militants were using Australian recruits as "cannon fodder".

About 70 Australians are still believed to be fighting in the Middle East, while another 20 have returned home.

Australia has introduced legislation to combat the threat from those fighters who come back home.

Propaganda had fooled Australian recruits who believed they were playing an important role in a religious crusade, Mr Brandis said.

"They are simply using them as frontline cannon fodder, suicide bombers and propaganda tools."

Among those reportedly killed in Syria is Sydney man Mohammad Ali Baryalei, an alleged IS recruiter who was accused of masterminding a terror plot to behead random members of the Australian public.

Under a Foreign Fighters Bill that adopted by the Australian parliament in October, it is an offence for Australians to visit areas declared as "terror hot-spots".

Last week, Foreign Minister Julie Bishop used this provision to declare it an offence for Australians to visit the de-facto IS capital of Raqqa in Syria without a valid reason.

Australians face up to 10 years in prison for illegally visiting the region.