Earlier this month, an alert immigration checkpoint officer discovered that two men who arrived by ferry and were about to enter Singapore were on their way to Turkey and Syria, Prime Minister Lee Hsien Loong said.

They had tickets to Turkey, and were sent back to Indonesia, where the local police questioned them before releasing them, he added, in comments that underline the seriousness of the threat that terrorism poses to South-east Asia.

Mr Lee cited this incident in an interview in Singapore last Friday with Mr Greg Sheridan, foreign editor of The Australian newspaper, that is due to be published today.

A transcript of the interview was released to Singapore media.

Mr Sheridan had asked whether the Syria imbroglio was making the threat more acute, with some 700 South-east Asians fighting with terror group Islamic State in Iraq and Syria (ISIS).

PM Lee replied that, while the issue of extremism had been around before the Sept 11, 2001 attacks on the United States, its latest incarnation is "very serious" as ISIS has territory and seduced hundreds from the region to travel there.

"A few from Singapore are there and close to a dozen have tried to go there. We have stopped them and headed them off," he said. He noted that ISIS had managed to attract members of the military: Malaysia recently arrested two commandos for ISIS links, and about a dozen personnel have such links.

While the number of Indonesians with ISIS is a small proportion of the population, it is a big enough number to cause trouble. At the same time, some terrorists in jail have pledged allegiance to ISIS. Several hundred are due to have their terms run out this year or next.

Mr Lee was also asked why such ideology had proven so persistent and difficult to counter.

"It is a very difficult problem. It is not purely religion and yet it is not unrelated to a certain warped view of religion," he said. "Some people genuinely persuade themselves that this is the way to Heaven and so they pursue this perverted path. Others know very little about religion or doctrine. Something has gone (wrong) with their life and this is their way to hit out at the world or at their society."

Others are young people at the soul-searching stage of their lives who get misled, he said. Singapore has picked up students who were still in school, became interested in the ideology and got in deeper and deeper despite having no network or radical friends.

He also noted that, while about 70 people have been detained since 2001, over three-quarters have been released. Most have stayed clean; one has relapsed.

"There are a few whom I do not know how we will ever release them, or how we will release them for a very long time because they are very hardcore," he said, noting that the US had not been able to close Guantanamo Bay.

Lim Yan Liang