Indonesian authorities have arrested six suspected militants on Batam island who were believed to be linked to the Islamic State group and plotting an attack on nearby Singapore, an Indonesian police spokesman said.
Singapore said it had stepped up security in response.
Indonesian police spokesman Agus Rianto told reporters the suspects had been plotting with a member of the Islamic State militant group in Syria to attack Singapore via Batam, which is about 15km south of Singapore.
"What we understand so far is that they were planning to attack vital objects, busy areas including police offices," Rianto said.
The six suspects were suspected to have links to Bahrun Naim, an Indonesian fighting for Islamic State in Syria, police said.
Singapore said it had stepped up its internal and border security.
"This development highlights the seriousness of the terrorism threat to Singapore," a spokesman for Singapore's interior ministry said in a statement.
"The public are advised to remain vigilant."
Indonesian investigators believe that Naim was one of the masterminds behind an attack in January in the Indonesian capital of Jakarta, in which eight people were killed including the four attackers.
"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," Rianto said.
Security officials fear that Naim and other Islamic State leaders were now asking supporters in Indonesia and other countries to launch attacks at home, instead of being drawn to the fight in the Middle East.
"One thing I think is clear is Bahrum Naim has been able to establish a lot of communication with a log of people through his social media network," said Jakarta-based security analyst Sidney Jones.
"This would be a departure for Bahrum Naim and his supporters if they were really thinking of attacking targets outside Indonesia," she said.
Southeast Asian militants fighting for Islamic State in the Middle East have said they have chosen one of the most wanted men in the Philippines to head a regional faction of the radical group that includes Indonesians and Malaysians, security officials said last month.