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 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 will celebrate their National Day holiday on Tuesday.
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 ofpreparations 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."