Indonesian police on Friday arrested the six suspected militants in Batam, including the group's leader who police said had been plotting the launch.
"Our security agencies have been coordinating closely with the Indonesian authorities since the discovery of this attack plot, to monitor the activities of the group and to apprehend those involved," the Ministry of Home Affairs said in a statement. "We are grateful for the good cooperation by the Indonesian authorities and their actions to apprehend the group," it said.
The ministry said the police and other agencies have stepped up inland and border security measures in response to the threat. The attack plot "highlights the seriousness of the terrorism threat to Singapore", the ministry said, adding that Singaporeans are advised to remain vigilant.
Indonesia's national police spokesman Boy Rafli Amar earlier told The Straits Times the ringleader, Gigih Rahmat Dewa, 31, and Bahrun Naim, an Indonesian militant believed to have been fighting alongside the Islamic State in Iraq and Syria (ISIS) militant group, "previously planned to launch a rocket from Batam to Marina Bay Singapore”.
Boy Rafli did not provide details.
The group, whom the police have identified only as KGR @ Katibah GR, is accused of harbouring two Uighurs, one of whom is linked to the suicide bomber who tried to attack a police station in the city of Solo in Central Java on July 5.
The bomber, Nur Rohman, had taken the Uighur, identified only as Ali, from Batam to Bogor and then to Bekasi, where the latter was left with an Indonesian militant named Arif Hidayatullah, alias Abu Musab.
They were both arrested on December 23 in Bekasi on the outskirts of Jakarta. Ali was being primed as a suicide bomber in an attack planned for that same month. Arif is now in a maximum detention facility in Kelapa Dua, located not far from Jakarta.
The second Uighur, identified as Doni, has since been deported.
Nur Rohman, a 30-year-old Solo native, learned to build bombs from Bahrun, who remains in close contact with domestic terror cells and militants in Indonesia. He received instructions on how to assemble the improvised explosive device (IED), which contains ball bearings and screws, he used in his suicide attack from Bahrun via smartphone messaging app Telegram, a source said.
Gigih, meanwhile, is suspected to have received and channelled funds for radical activities from Bahrun, Boy Rafli added.
He also helped Indonesian Muslim "brothers" to travel to Syria to fight with ISIS via Turkey, with the assistance of Indonesians in the latter country, Mr Boy Rafli said.
Indonesia security forces are focusing on domestic terror networks and smaller militant cells trying to take over the mantle from Jemaah Islamiyah.
ISIS has also started to bare its teeth in the country through local proxies who have made no secret that police officers are among their top targets.
Indonesian intelligence officials are tracking the movements of Uighurs in the Riau Islands, which comprises Batam and other islands, over fears that they may join local militant groups and mount attacks in the country.
Militants from the Muslim ethnic group in China have been known to make the long journey from the far western Chinese region of Xinjiang to join Islamist extremists in Indonesia.