Singapore was aware of the group of militants based in Batam who were planning to attack Marina Bay, Home Affairs and Law Minister K. Shanmugam said yesterday.

The news that such a plot was being hatched so close to Singapore's borders was no surprise, he added in reply to media queries after the Indonesian police arrested six members of the militant cell in Batam.

Mr Shanmugam had, in recent months, raised concerns about terrorists targeting Singapore from abroad, and who could try to enter the country or locate themselves just outside the country.

"Our small size increases these risks," he said yesterday.

Apart from plotting the attack, the cell's leader, 31-year-old Gigih Rahmat Dewa, was helping Indonesian fighters travel to Syria and was receiving funds from Syria-based militant Bahrun Naim for attacks.

Both Mr Shanmugam and Deputy Prime Minister Teo Chee Hean, who is Coordinating Minister for National Security, noted that Singapore's security agencies and their Indonesian counterparts had coordinated closely to monitor the activities of the group and apprehend those involved.

The ministers said they were grateful for Indonesia's cooperation and enforcement action.

Mr Teo said on Facebook that the police and other agencies have stepped up security measures in Singapore as well as at its borders.

"This development highlights the seriousness of the terrorism threat to Singapore, and the importance of the SG Secure national movement," he added.

Prime Minister Lee Hsien Loong will launch SG Secure next month. It aims to mobilise Singaporeans to be more conscious of security, respond to a terror attack and stay united as a society after an attack.

Defence Minister Ng Eng Hen said in a Facebook post that the arrests are a reminder that Singapore has to stay vigilant and keep improving its defence systems.

"Since 9/11, the SAF has paid very close attention to aerial threats and has upgraded our air defence systems," he added, stressing the need for people to stay united.

Security analysts like Mr Jasminder Singh of the International Centre for Political Violence and Terrorism Research said the arrests are a reminder of the need to stay vigilant in the face of the terror threat.

Mr Singh said that while the actual risk of a successful rocket attack on Marina Bay is debatable, the threat of terrorism remains real given Singapore's strong stance against terrorism and the Islamic State in Iraq and Syria (ISIS) group.

Referring to remarks Mr Lee made on an official visit to the United States this week, he said: "As Prime Minister Lee mentioned, Singapore being a target is not new - whether or not we are part of the US-led coalition against ISIS - because of what we represent: A harmonious multiracial and multi- religious country."

The arrests underline the importance of intelligence sharing among governments in the region, he noted. But a population that is alert to suspicious and radical activity is also needed to deter an attack.

This is why Ms Susan Sim, vice-president for Asia at security consultancy The Soufan Group, said SG Secure is crucial.

"It is a way to teach us to practise situational awareness so we don't remain complacent or become paranoid, but can contribute meaningfully to keeping ourselves and everyone else around us safe," she said.

Singapore's low crime rate and high level of safety may lead some to assume that attacks may not happen here. But Ms Sim noted that recent incidents in Germany and France, where attackers used knives and a truck respectively, show "the modus operandi of a terrorist is limited only by the imagination".

"You don't need sophisticated tools to launch an attack," she said.

Reverend Bobby Lee, secretary of the Geylang Serai Inter-Racial and Religious Confidence Circle, said people need to realise that today's peace "has to be guarded".

"The public needs to realise that it is not just the police's duty to protect us. We can't think that we'll just leave such matters to the experts," he said. "We have to have fortitude, be aware and remain vigilant."