Jakarta - Indonesian police on Friday arrested six suspected militants over a plot to fire a rocket at an upmarket Singapore waterfront district from a nearby island, prompting the city-state to tighten security.The men, aged between 19 and 46, were detained by elite anti-terror police on the Indonesian island of Batam, which lies just south of the affluent city-state.The alleged leader of the Indonesian group is accused of planning the attack with a leading Indonesian militant who is now believed to be fighting with ISIS group in Syria.It was the latest terror plot in the world's most populous Muslim-majority country, where there has been a surge in attacks and attempted attacks this year due to the growing influence of IS.The pair "planned a terror attack in Singapore. They wanted to attack Singapore with a rocket from Batam," national police spokesperson Agus Rianto told reporters.Police said the target was Marina Bay, a district that is home to Marina Bay Sands, a luxury complex that includes shopping malls, hotels and a casino.Rianto added police had "preliminary data" and were still investigating the plot, and named the alleged ringleader as 31-year-old Gigih Rahmat Dewa.Analysts said it was unclear whether the militants had the ability to carry out such a plan, which would involve firing a rocket over a distance of about 20 kilometres. Singapore said it was aware of the plan and security had been stepped up inland and at the city-state's borders."This does not come as a surprise," said Home Affairs Minister K. Shanmugam."I have spoken several times, about plans being made in places just outside Singapore, to target Singapore - we were serious about the threats."Growing ISIS support Sidney Jones, director of Jakarta think-tank the Institute for Policy Analysis of Conflict, told AFP it was necessary to wait for further information about the plot before drawing firm conclusions.But she added: "I have no idea whether there was any capacity to do this. I think it highly unlikely that the plan had got very advanced."There have been signs of support for ISIS in Singapore.Singapore in recent weeks jailed four Bangladeshi workers accused of planning to join IS for raising money to fund attacks in their homeland, and also detained an Australia-based Singaporean who allegedly glorified the jihadists and backed the establishment of a caliphate in the city-state.Police suspect Dewa, 31, received and distributed funds sent by Naim. Naim has been linked to several recent terror plots in Indonesia, including a suicide bomb attack on a police station in the city of Solo last month that left one police officer injured.Dewa is also accused of previously harbouring two members of China's ethnic Uighur minority, some of whom have travelled to Indonesia to join militant groups, and of helping extremists on their journeys to Syria.Indonesia has long struggled with Islamic militancy and has suffered a string of attacks in the past 15 years, including the 2002 Bali bombings that killed more than 200 people.A crackdown had weakened the most dangerous networks but ISIS has proved a potent new rallying cry for the country's radicals, and hundreds of Indonesians have headed to the Middle East to join the jihadists.In January ISIS-linked militants launched a deadly gun and bomb attack in Jakarta which left four attackers and four civilians dead.