By Kanupriya Kapoor

BATAM, Indonesia (Reuters) - Indonesian police found only a stash of bomb-making materials after arresting six men suspected of planning a rocket attack on Singapore from a nearby island, a senior officer said on Saturday.

But police spokesman Boy Rafli Amar said a search was on for more members of a gang now believed to be part of a wider network taking instructions from a Syria-based Islamic State militant to launch attacks across Southeast Asia.

Singapore called for heightened vigilance a day after the suspected militants were rounded up. It said their plot to target Marina Bay, a glittering downtown waterfront, showed the city-state needed to defend its multiple points of entry.

The six suspected militants were detained on Friday in dawn raids on Batam island, about 15 km (10 miles) south of Singapore, where police believe the men planned to fire the rockets from.

"There were no sophisticated arms confiscated, only bomb-making materials," Amar told Reuters. Asked whether the initial investigation indicated other groups were planning similar attacks, he said "Yes, there still are".

Singapore's home affairs minister, K. Shanmugam, said the men had plans to hit Marina Bay, a popular entertainment area with a waterfront promenade, a giant ferris wheel and a swanky casino resort. The Singapore Formula One Grand Prix is held annually on the streets of the Marina Bay neighborhood.

"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 radicalized 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 will celebrate their National Day holiday on Tuesday.

SINGAPORE, A TARGET FOR ISLAMIC STATE

Authorities identified the leader of the group arrested on Batam as Gigih Rahmat Dewa, who local media said was a 31-year-old factory worker from the Javanese city of Solo. Solo has been linked to several previous attacks by Islamist militants in Indonesia.

The group was suspected of having direct links to Bahrun Naim, an Indonesian who had lived in Solo but is now believed to be fighting with Islamic State in Syria.

"The six people led by GRD had planned to launch attacks," National Police Chief Tito Karnavian told reporters, referring to Dewa by his initials. "They were in direct contact with Bahrun Naim in Syria and he had ordered them to attack Singapore and Batam."

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.

Police said they had not yet discovered any physical evidence of preparations for a rocket attack.

"We are currently studying what materials they had and I cannot say that a rocket was found," Batam District Police Chief Helmi Santika told Reuters. "Among other things, several weapons were seized including arrows, long-rang firearms and pistols."

TURNING POINT?

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. 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.

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," he said.

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 neighboring 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.

(Additional reporting by Saeed Azhar in Singapore and Fergus Jensen and Agustinus Beo Da Costa in Jakarta and William James in London; Writing by John Chalmers and Randy Fabi; Editing by Tom Heneghan)