(Transcript from SBS World News Radio)

The federal government says it's prepared to take strong action against Australians who leave the country to support or fight with militants in Iraq or Syria.

Foreign Minister Julie Bishop says these people pose a significant national security risk if they return to Australia after being radicalised.

She says she's already cancelled a number of passports of Australians suspected of fighting with insurgent forces overseas.

The government is also indicating that it might consider cancelling the Australian citizenship of dual citzens actively fighting with the militants.

Amanda Cavill reports.

(Click on the audio tab above to hear the full report)

The Foreign Minister says there are believed to be about 150 Australians currently fighting with the militants in Syria or Iraq, and some have already had their passports cancelled on the advice of security agencies.

She says in Syria, it seems the group has moved from supporting the more moderate opposition groups to the more extreme ones and that includes the group ISIS, or the Islamic State of Iraq and Syria.

The Sunni militant group has taken over key cities in northern Iraq and is within striking distance of the capital Baghdad.

Ms Bishop says the government is concerned about the threat to national security if the Australian fighters come back to Australia.

"We are deeply concerned about Australians, including dual nationals, who are supporting or working with or actually fighting with this extremist group. These are brutal people. The executions and the killings and their boasting of it on social media makes this a particularly virulent form of terrorism. And these people are so extreme that al-Qaeda is even distancing itself from them. Now, we are concerned that Australians are working with them, becoming radicalised, learning the terrorist trade. And if they come back to Australia, of course, it poses a security threat.

It's illegal under Australian law for any citizen to fight with any anti-government forces overseas.

Prime Minister Tony Abbott says the government will be trying to stop any Australians who have fought in Syria or Iraq from re-entering Australia.

But he says if these people do manage to return, they will be arrested.

"They will not come back and if they come back they will be taken into detention because our community will be kept safe by this government. We are keeping as careful a watch on all of these individuals as we possible can. Be in no doubt that some individuals from this country are now participating in acts of barbarity in Iraq. These people should have no place in our country and and we will do our best to keep them out. And if they can't be kept out they will be taken into detention."

The previous Labor government cancelled 18 passports of Australians believed to have joined opposition forces trying to overthrow the Syrian government.

Labor's foreign affairs spokeswoman Tanya Plibersek has told the ABC such fighters pose a real risk to the Australian community if they return.

"When we were in government, the current government also, have taken steps to try to prevent people going to Syria to fight, cancelled passports and so on but of course some people have managed to go there. The speculation, this isn't confidential information, but the speculation is that it's 150 to 200 people, those people do return well trained, radicalised and with a degree of sick sort of street cred that allows them to convince other impressionable young people that perhaps going to fight is a good idea or perhaps committing crimes here in Australia might be a good idea.

Immigration Minister Scott Morrison says he's looking at other measures that can be taken to protect Australia.

Mr Morrison says revoking citizenship can not be ruled out.

He says militants living in Australia as permanent residents can expect to have their visas revoked.

"Nothing is ever ruled out on these matters and they're always subject to continued discussion. Issues of people who are on visas is a very different matter and the 501 powers, I think are very clear. As you know, we've just used those powers in relation to Mr Alex Vella, the President of the Rebels motorcycle club. We take these matters very seriously. So, whether it's those like Mr Vella or whether it's terrorists or others engaged in these sorts of acts, then they can expect me to do what they would expect me to do."

Iraq's ambassador to Australia, Mouayed Saleh, says Australians who fight for ISIS should be stripped of their citizenship.

"If they join ISIS, absolutely. The whole world would not allow people to join these savage forces."

The government says it has also shown its commitment to ensuring the security of Australia and its citizens by restoring more than $700 million in funding for border protection cut by the previous Labor government.