Six suspected militants were rounded up on Friday in dawn raids on Batam island, which is about 15 km south of Singapore.

Singapore called for heightened vigilance on Saturday, a day after Indonesian police arrested a group of men they believed were plotting a rocket attack on the wealthy city-state with the help of a Syrian-based Islamic State militant.

The six suspected militants were rounded up on Friday in dawn raids on Batam island, which is about 15 km south of Singapore, and local police chief Sam Budigusdian said they were still being held there while investigations continued.

Singapore's Home Affairs Minister, K. Shanmugam, said the men had plans to hit Marina Bay, the state's glittering downtown waterfront, where night-time Formula One Grand Prix Races are held alongside a giant ferris wheel and a swanky casino resort.

“This shows how our enemies are thinking of different ways of attacking us,” Shanmugam said in a Facebook posting.

“Terrorists ... will seek to come in through our checkpoints; they will also try to launch attacks from just outside. And this is in addition to lone wolf attacks from radicalised individuals/groups. We have to be extra vigilant.”

Batam is linked to Singapore by frequent ferries and its beach resorts and golf courses are a popular weekend getaway destination for Singaporeans, who are preparing to celebrate their National Day holiday on Tuesday.

Singapore, a target for IS

Multi-ethnic Singapore, a major commercial, banking and travel hub that is home to many Western expatriates, has never seen a successful attack by Islamist militants.

However, authorities did break up a plot to bomb several embassies soon after the Sept. 11, 2001, attacks on the United States, and a Singaporean militant was accused of plotting to crash a hijacked plane into the city's airport in 2002.

Prime Minister Lee Hsien Loong said this week Singapore was a target for Islamic State because it is “a rational, open, cosmopolitan country” even though it is not involved in the U.S.-led campaign against the group in the Middle East.

The six arrested on Batam - mostly factory workers, according to local media - were suspected of having links to Bahrun Naim, an Indonesian now fighting with Islamic State in Syria.

“There's a link to Bahrun Naim because there was communication with Bahrun Naim - but whether they were affiliated with Bahrun Naim's group or not - this is what we're investigating now,” police spokesman Agus Rianto said.

Indonesian investigators believe that Naim was one of the masterminds behind an attack in January in the Indonesian capital, Jakarta, in which eight people were killed, including the four attackers.

In a blog post after the coordinated shootings and suicide bombings across Paris last November, Naim urged his Indonesian audience to study the planning, targeting, timing and courage of the jihadis who killed 130 people in the French capital.

Turning point?

Asked on Friday if physical evidence of preparations for a rocket attack had been discovered, Rianto said “not yet".

Some security analysts were doubtful that a rocket attack on Singapore from Batam was feasible, but Tim Ripley of Jane's Defence Weekly said it was possible.

“They would be the long-range variants of the Grad rocket -originally from Russia but copied in China, Iran, Pakistan and several other countries,” Ripley said. “Very simple to use but very inaccurate at the ranges for this attack. The damage would depend on where they hit but the potential for casualties would be high.”

If the plan for a rocket launch on Singapore is confirmed, it would suggest that militants in Southeast Asia are preparing far more sophisticated attacks than those of recent months.

Indonesia, home to the world's largest Muslim population, has itself seen attacks by Islamist militants before. The bombing of two nightclubs on the resort island of Bali that killed 202 people was among a spate of attacks during the 2000s.

Police were largely successful in destroying domestic militant cells after that, but they now worry the influence of Islamic State will bring a resurgence of jihadi violence.

Authorities in Indonesia and neighbouring Malaysia say dozens of men have gone from those countries to join Islamic State's fight in the Middle East. Security officials fear that Naim and other Islamic State leaders are now asking supporters in Southeast Asia to launch attacks at home.

However, Jakarta-based security analyst Sidney Jones said it would be a departure for Naim and his supporters if they were thinking of attacking targets outside Indonesia.

“One thing I think is clear is Bahrun Naim has been able to establish a lot of communication with a lot of people through his social media network,” Jones said.