The authorities have been quietly acting on information about "definite" plots by terrorists to attack targets in Singapore, Prime Minister Lee Hsien Loong revealed in his National Day Rally speech yesterday.

In the latest, clearest reminder that Singapore remains firmly within terrorists' sights, Mr Lee said some stepped-up security measures have been taken in response to what security agencies consider "a real threat" based on intelligence.

He did not elaborate but one such plot was made public earlier this month, when Indonesian police arrested factory worker Gigih Rahmat Dewa, 31, and four other members of his terror cell in Batam.

Gigih had been in touch with Syria-based Indonesian ISIS militant Bahrun Naim to plan a rocket attack on Marina Bay. (ISIS stands for Islamic State in Iraq and Syria.)

"Gigih's plot to attack MBS is not the only definite plan by terrorists to attack targets in Singapore that we know of," said Mr Lee.

"We know there have been others, we've quietly acted on the information, we've taken precautions."

These include stepped-up patrols and increased protection for major events and prominent premises. The authorities have also shifted and rescheduled certain events because of these threats, he added.

"We take it seriously - it's not certain enough to warn everybody publicly, but serious enough for us to act quietly.

"So the next time you see a patrol in the city, or some extra security in some areas, it may be we are just taking precautions, or doing a show of force as deterrence," said Mr Lee. "But it could also be in response to a real threat that we've heard about."

Within the region, terror groups are active and have shown their capacity to carry out attacks, he said.

Despite a crackdown by the Indonesian police last December, one cell evaded the dragnet to mount a January attack in central Jakarta.

In June, Malaysia suffered a grenade attack in Puchong, and the authorities there have arrested some 200 ISIS supporters, including at least 13 armed forces personnel.

Malaysia also found seven prison wardens radicalised by the detainees they were handling, and they had to themselves be deradicalised.

Terror group ISIS continues to court South-east Asians to join its extremist cause, Mr Lee added, noting that some 1,000 Malaysians and Indonesians are fighting for the group in the Middle East, enough for ISIS to form Bahasa Indonesia-speaking battalion Katibah Nusantara.

The group also put out a Bahasa Indonesia magazine, Al Fatihin, that Singapore banned last month.

Mr Lee said Singaporeans are not immune to such propaganda.

He noted that a dozen radicalised Singaporeans have been arrested over the past two years.

Several tried to go to the Middle East to join ISIS, and a few were prepared to mount attacks in Singapore. One had planned to kill the President or Prime Minister if he could not get to Syria.

Mr Lee noted that some of those detained had surfed radical websites, and others had listened to extremist radio stations in the region.

Last Friday, the authorities dealt with four Singaporeans who had become radicalised and intended to travel to Syria to join ISIS.

"We continue to pick up a steady trickle of such people, one or two a month," Mr Lee said.

Fortunately, Singapore has a font of strength in fighting the forces of intolerance and extremism, he added: Communities and their leaders innately understand the importance of multiracialism, the need to be inclusive, and to compromise so as to maintain social harmony.

And this give-and-take has to be nurtured at a time when terrorism threatens the social fabric, he said.