SYDNEY (Reuters) - Australian police on Thursday arrested a man believed to have fought with Syrian militants and a youth who was stopped at the border allegedly planning to join Islamic State in Syria.

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The pair were detained in early morning raids in Sydney, underscoring Australia’s tough stance on dealing with so called “foreign fighters” heading overseas to join in conflicts in countries including Syria and Iraq.

Australia has imposed some of the harshest penalties in the world on foreign fighters returning home as the country leads a push for the United Nations to adopt international standards to curb the threat they pose.

“It is very unfortunate that even though ISIS is losing significant ground over there that we still have people, particularly young people, that are attracted to the ideology and are still attempting to leave the country,” Neil Gaughan, national manager of counter-terrorism for the Australian Federal Police, told reporters.

The 24-year-old man and the 17-year-old youth were both charged with breaching Australia’s foreign incursion laws, which prohibit travel to foreign countries with the intention of engaging in hostile activity. Those found guilty face up to 25 years imprisonment.

Security analysts have put the number of foreign fighters in Iraq and Syria, traveling from scores of countries around the world, in the thousands. The Australian government estimates more than 100 nationals are among them.

A staunch U.S. ally, Australia has been on heightened alert for attacks by home-grown radicals. Recent “lone wolf” attacks include the 2014 Sydney cafe siege in which two hostages and the gunman were killed and the murder of police accountant Curtis Cheng by 15-year old Farhad Khalil Mohammad Jabar in March 2016.

Police said the 24-year-old man traveled to Syria in 2013, where he joined al Nusra, an affiliate of al Qaeda, before returning to Australia in 2014.

The 17-year-old male attempted to fly to Syria in January 2015 to join Islamic State but was stopped by police before he could board the plane, officials said.

“The disturbing feature that we are seeing increasingly is very young people who have not been on the radar screen of any of our current terrorist agencies, state or federal, are becoming very quickly radicalized and either attempting or undertaking terrorist acts,” Prime Minister Malcolm Turnbull told reporters in Sydney.