Two teenage brothers intercepted by authorities at Sydney Airport were suspected of trying to head to Syria to join Islamic State (IS) fighters.

Federal Police said the boys aged 16 and 17 were detained on Friday on suspicion of attempting to prepare for incursions into foreign countries for the purpose of engaging in hostile activities.

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They said the Sydney brothers were "subsequently unarrested and released into their parents' custody" and would not comment further because investigations were "ongoing".

But sources have told the ABC their luggage contained extremist paraphernalia and a letter on how to fabricate a cover story to authorities when leaving the country.

The ABC has also learnt the teenagers were headed for Turkey, a common transit point for those wanting to join IS.

"Turkey of course is the bordering country for Syria and it's the usual jumping off point for foreign fighters going to Syria to join Islamic State," terrorism expert Professor Clive Williams of Macquarie University said.

'Kids, not killers,' says Immigration Minister

Earlier the ABC reported customs officers determined that they were intending to travel without the knowledge of their parents.

Immigration Minister Peter Dutton said the teens were "kids, not killers".

Mr Dutton said the boys were stopped on their way to a potentially very dangerous situation, and their parents were "shocked" to learn of their plans.

"These two young men, aged 16 and 17, are kids, not killers," he said.

"They shouldn't be allowed to go to a foreign land to fight and to come back to our shores eventually more radicalised.

"In some cases, these young people who are going off to fight in areas like Syria will be killed themselves, and that's a tragedy for their families, for their communities, and for our country.

"We have to be absolutely determined to stare down this ever increasing threat."

Prime Minister Tony Abbott thanked the border protection officers for doing their job, saying the boys had been prevented from doing something "very dangerous".

He said there were about 400 people in Australia that security agencies were currently monitoring.

"We know that there are other people who might be tempted to do silly things, and these are two youngsters who ... suddenly got it into their head to go and do something that is very dangerous for them and potentially very dangerous for others," he said.

"I'm pleased to say that the new enhanced screening procedures at our airports, and the anti-terrorist teams that we've now got at our airports, appear to have done a very good job.

"What we don't want is people coming into this country with evil intentions, and we don't want people leaving this country with evil intentions."

The brothers were later allowed to leave the airport with their parents, who were unaware of their plans.

Initially Immigration Minister Peter Dutton said the pair were issued with court attendance notices but the AFP later clarified that was not the case.

Internet propaganda heightens risk of radicalisation: Dutton

Mr Dutton said information young people could access on the internet meant they could be more inclined to travel to conflict zones.

"Traditionally, parents of teenage children had to worry about alcohol, drugs, and driving and now there is an added threat of the information being delivered to them over internet, particularly if they're inclined to go and fight in a foreign land," he said.

"[And] most importantly for our country, when they return, they return more radicalised, and [are] a greater threat to the Australian public.

"We have to be absolutely determined to stare down this ever increasing threat."

The Foreign Fighters Bill passed by Parliament last October makes it illegal to travel to areas declared as terrorist zones, without a specific humanitarian or family purpose.

Under the bill, Customs officers are allowed to detain people at the airport if they believe they may be travelling to one of the prohibited areas.

Foreign Minister Julie Bishop has listed both the province of Al-Raqqa in Syria and Mosul in Iraq as off-limits.

Australians found to be illegally visiting the regions could face up to 10 years' prison.