Around 150 Australians have fought with radical militants in Syria and Iraq, raising government fears of a terrorist threat to Australia if the fighters return home, the foreign minister said on Thursday.

Foreign Minister Julie Bishop said she had cancelled a number of passports on the advice of security agencies in a bid to reduce the security threat to Australia. She did not say whether the passports were cancelled to prevent Australians leaving or returning.

"Our best estimate is that there about 150 Australians ... who have been or are still fighting with opposition groups in Syria and beyond," Bishop told Australian Broadcasting Corp.

'We are concerned that Australians are working with them, becoming radicalized, learning the terrorist trade.'- Australian Foreign Minister Julie Bishop

"In Syria, it seems that over a period of time they have moved from supporting the more moderate opposition groups to the more extreme and that includes this brutal extremist group ISIS," she said, referring to the Islamic State of Iraq and Syria, the al-Qaeda splinter group leading Sunni militants in Iraq. It is also known as Islamic State of Iraq and the Levant, or ISIL.

Australian police have charged several people in recent months with recruiting fighters and supporting the insurgency in Syria. Several people have been prevented from leaving Australia on suspicion that they intend to fight.

"We are concerned that Australians are working with them, becoming radicalized, learning the terrorist trade and if they come back to Australia, of course it poses a security threat and we're doing what we can to identify them," Bishop said.

Australia sent 2,000 troops to support U.S. and British forces in the 2003 Iraq invasion. Bishop said neither the United States nor Iraq had yet asked for Australia's help in the latest crisis.

But she said Australia would give $5.1 million to the United Nations to provide food, medical assistance, tents, access to clean water and hygiene kits to hundreds of thousands of people fleeing violence in north and northwest Iraq.