The conflict in Syria now in its third year is a security concern for many countries, including Singapore, deputy prime minister Teo Chee Hean told Parliament on Wednesday.
The government already knows of Singaporeans who have gone there to take part in the conflict or who have plans to do so, he added.
He revealed that a Singaporean woman is believed to have gone to Syria with her foreign husband and their two teenage children.
"The whole family is taking part in the conflict in various ways, either joining the terrorist groups to fight, or providing aid and support to the fighters," he said.
Earlier this year, the home affairs ministry disclosed it was investigating another Singaporean, Haja Fakkurudeen Usman Ali, 37, for allegedly going to Syria with the intention of taking part in armed violence there.
Teo said that the naturalised Singapore citizen of Indian origin had also taken his wife and three children, then aged between 2 and 11, with him.
Several other Singaporeans had also intended to take up arms in Syria, but were detained before they could do so. There are also others who have "expressed interest" to do so, who are under investigation.
In Malaysia and Indonesia, there had also been those who joined the armed conflict in Syria and Iraq, which is being rocked by an uprising of an extremist Sunni group trying to carve out a purist Islamic state across both sides of the Syria-Iraq border.
Teo said: "The presence of former foreign fighters in our region - whether they originate from Southeast Asia or elsewhere, is a security threat to us. This threat is magnified if these returnee fighters are Singaporeans."
Drawing parallels with the Soviet-Afghan war in the 1980s, he said that the al-Qaeda terrorist organisation was spawned from that conflict, which had also attracted scores of foreign fighters.
And the al-Qaeda had planned attacks on Singapore after the September 11 terrorist attacks in the US in 2001, through the regional terrorist organisation Jemaah Islamiyah (JI), he noted.
"Foreign fighters in Syria may similarly return from conflict proficient in terrorist activities in their home countries or overseas or provide logistical help to those they have befriended in Syria," said Teo.
Another worry is the impact of this on social cohesion, he added.
If support for the fighting in Syria becomes more widespread, he said, it could cause "disquiet on the ground" and mistrust between different communities.
To guard against these threats, Singapore will "continue to investigate persons who intend to engage in violence overseas, so as to prevent them from posing a security threat to Singapore and their fellow citizens", said Teo.
The government will also work with religious leaders and community groups to counter the radical propaganda used by terrorists to recruit fighters.
Said Teo: "At the core of the issue is an ideological battle, between those who distort Islam for their violent political ends, and those who uphold the tenets of Islam as a religion of peace."
His speech comes amid growing concern about foreigners, especially from Europe and the United States, travelling to Syria to join rebels in their fight against President Bashar al-Assad's regime.
Teo noted that the internet and social media had been a "game-changer" in the conflict, allowing extremists to market their cause and also recruit foreigners to fight.
Some extremists have also been posting selfies online, attracting young people through the "jihad cool" factor, he said.
He urged Singaporeans to keep a lookout for family members and friends, and to bring them to the attention of authorities if there are any signs of them becoming radicalised.
"By intervening early...(we) would be saving these individuals from taking a course of action that would have caused them and others harm," he said.
Singaporeans who want to help Syrian civilians who are victims of the violence, should check with the Islamic Religious Council of Singapore, or Muis, to see if humanitarian organisations they are donating to are bona fide, and not just fronts for extremists to raise funds, he added.
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