SINGAPORE - Singapore is very worried about the effectiveness with which terrorist group Islamic State in Iraq and Syria (ISIS) has used social media and the Internet to indoctrinate and recruit people from this region, Prime Minister Lee Hsien Loong has said.

Speaking just days after news broke that a 51-year-old Singaporean, Mustafa Sultan Ali, had been detained after attempting to join the conflict in Syria, Mr Lee said that ISIS has been able to "very intelligently" use social media and the Internet.

He noted that these tools were less available to terrorist groups of an earlier time, such as Al-Qaeda, which was responsible for the 9/11 attacks.

"ISIS has been able, very intelligently, to use the internet and social media to reach out, to indoctrinate, to recruit and to subvert - including in Malaysia, Indonesia, and Singapore," he said.

"We are very worried, we take it very seriously."

He was speaking to former ambassador Chan Heng Chee, now the chairman of the Lee Kuan Yew Centre for Innovative Cities, in an interview which will be televised on Sunday.

ISIS has attracted 30,000 foreign fighters to territory it controls in Syria and Iraq, including about 1,000 from South-east Asia.

PM Lee noted that Singapore has a good programme run by local Malay/Muslim leaders that helps rehabilitate those influenced by extremist ideology, but that it is not possible to identify everyone at risk.

"We have a good programme with the Religious Rehabilitation Group, working with people who have been led astray, people who have been detained. Working with their families, helping their families to see through a difficult time and working with the community, so that the community is not led astray," he said.

"But I think no matter how good our programme is, it is not possible for us to identify every last person, who typed ISIS on Google and found some link, some preacher, and got led astray. We just have to be vigilant and work hard at it (and) hope that we catch them earlier."