SYDNEY (AFP) - Australia on Sunday said it stopped two teenage brothers at Sydney Airport believed to be heading to the Middle East to fight, amid growing concern in Western countries over young people joining militant groups.

Immigration Minister Peter Dutton said the two boys, aged 16 and 17 and from Sydney, had tickets to an undisclosed Middle Eastern country and raised the suspicions of customs officers on Friday night.

"These two young men... are kids, not killers, and they shouldn't be allowed to go to a foreign land to fight and to come back to our shores eventually more radicalised," Mr Dutton told reporters. "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."

The minister said a search of the boys' luggage raised more questions about their trip and they were referred to the federal police's counter-terrorism unit. He said the two youths "had taken a very radical decision ultimately without the knowledge of their parents". "Their parents, as I understand it, were as shocked as any of us would be."

An Australian Federal Police spokesman said in a statement that the boys, whose identities were not released, were "arrested under suspicion of attempting to prepare for incursions into foreign countries for the purpose of engaging in hostile activities".

They were later handed back to their parents and an investigation is ongoing, she said.

The case came as the families of three British schoolgirls who left their London homes to join Islamic State in Iraq and Syria (ISIS) militants in Syria in February criticised the authorities for not warning them their children risked being radicalised.

Mr Dutton would not say if the teens were linked to ISIS. About 100 Australians were fighting with ISIS and other terrorist groups in Syria and Iraq, with another 150 supporting them at home, Prime Minister Tony Abbott said last week.

Australia announced on Tuesday it was sending another 300 troops to Iraq in a joint mission with New Zealand to help train local forces fighting to reclaim territory seized by ISIS.

The nation has stepped up its security measures amid fears of heightened threats from "home-grown" ISIS-inspired extremists, including revoking citizenship for dual nationals linked to terrorism.