The concern about home-grown terrorism has taken an alarming twist today with death threats being made against a Sydney Muslim leader who spoke out against Australian radicals.

Dr Jamal Rifi and his family have been forced to take extra security measures in response to online threats that were posted on Twitter.

Muslim community leaders say combating radicalism in the community is harder than ever, in part because of the power of social media.

Dr Rifi has publicly denounced Australian terrorists Khaled Sharrouf and Mohamed Elomar. NSW Police have issued warrants seeking the immediate arrest of both men for terrorism offences if they return to Australia.

Earlier this week Dr Rifi said an image on posted on Twitter, reported to be Sharrouf's seven-year-old son holding the severed head of what is said to be a Syrian government soldier, caused more harm to the cause Sharrouf is claiming to fight for.

"I believe what he is doing now is only feeding his own ego and demented attitude towards life," Dr Rifi said.

"Our community is totally upset with him and totally against IS ideology who are [using] propaganda to recruit young Australians into a war that is not their own to fight."

He says is is alert "but definitely not alarmed" by the threats made against him.

"I'm taking that threat very seriously because I can take care of myself, I can take precaution but when it involves my kids, I need to ensure their safety and security," he said.

"They labelled me as a dog and in their own twisted ideology, that means I am outside of the Islamic fold and my killing is permissible."

Dr Rifi says "one radical is one radical too many".

"We have worked tirelessly over the last couple of years about immunising our kids from those damaging ideas and radicalisation and I think we have done a pretty good job," he said.

"Nevertheless, we can't actually police every kid, what happened at their home, in front of their computers.

"It is much harder now [than it was in the post 9/11 era] with the proliferation of the social media and internet, but we need to see it as a battle of ideology and it is in our best interests as Muslims not to allow these people to hijack our religion."

Authorities concerned by online glorification of extremists

Counter-terrorism experts are among those who are concerned that the online glorification of extremists is inspiring impressionable youngsters who are already on the outer.

"It's something that's on everyone's radar and you will never be able to do away with that threat and we've seen many instances particularly in the US for instance," said the Deputy Commissioner of New South Wales Police, Nick Kaldas.

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"If somebody's inclined to sit by themselves in a home or a unit, get on the internet and work out how to do bombings and how to plant things and how to put together explosive devices and then he goes and does it without discussing it with anyone, obviously that's very, very difficult to detect.

"The biggest threat we face I think in terms of terrorism is complacency.

"The biggest change has been is that the conflict zones in places in the Middle East seem to be more frequent and more accessible and perhaps through social media such as Twitter, Facebook and so on, much more visible to the naked eye.

"And if they are more visible, they're likely to attract more people's attention and not everyone who's attracted to look is thinking logical thoughts sometimes."

Deputy Commissioner Kaldas says the many outreach programs already up and running cannot work in isolation.

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"The online is more problematic in that its almost invisible. We cannot be all knowing, all seeing and that's where the community comes in and you're right, maybe we are traditional but we're certainly doing a whole lot more than we were perhaps 20 years ago," he said.

"The bottom line is if somebody's going to get on the internet and read inappropriate material and engage in inappropriate discussions which may or may not lead to violence, the best source of opposing that and stopping that is his own family and the community.

"We would ask all parents to be aware of what their kids are doing. I know they want to lock themselves in a room.

"If you've got a teenage son or someone in their early 20s who's living under your roof, I'd suggest you've got an obligation to at least talk to them about what they're doing and be aware of what business they're conducting behind the closed door."

He says everyone needs to take responsibility when radicals go overseas and then want to come home.

The young boy pictured holding the severed head is reported to be an Australian jihadist's seven-year-old son. ( Supplied )

"The community has a great ownership in this, the police at state level do, the government at state level does as well as obviously our federal colleagues and the Federal Government," he said.

"We've all got a hand in this and it's got to be all shoulders to the wheel."

The head of the Imams Council, Professor Ibrahim Abu Mohammed, told the ABC's Lateline program extremists "should be re-educated".

"We are working on this. We have methods that work on the correction of their ideas. We try to cure them. Our educators are young men who are native speakers, who are born here and who are specialised in this field," he said.

And he says those who do engage with militants overseas should not be banned from coming home.

"If we do, we are transforming them from possible terrorists to genuine terrorists. We should absorb them, rehabilitate them, re-educate them," he said.

"We should not deny them entry, as they will regroup elsewhere. If that happens, we are not fighting terrorism, but shifting it to another place."