Indonesian police on Friday arrested six suspected militants over a plot to launch a rocket attack on an up-market Singapore waterfront district, officials said.

Singapore confirmed it was aware of a plot and said security had been stepped up in the city-state.

The six men, aged between 19 and 46, were detained on the Indonesian island of Batam, which lies just south of the affluent city-state, said national police spokesman Agus Rianto.

The alleged leader of the group, Gigih Rahmad Dewa, is accused of planning the attack with Bahrun Naim, a leading Indonesian militant who later went to fight with the Daish militant group in Syria, Rianto said.

The pair "planned a terror attack in Singapore. They wanted to attack Singapore with a rocket from Batam," the spokesman told reporters.

Another police spokesman, Boy Rafli Amar, said the target was Marina Bay. The district is home to Marina Bay Sands, a luxury complex that includes shopping malls, hotels and a casino.

Singapore's Ministry of Home Affairs said in a statement it was "aware that plans were being made by the six terror suspects arrested by the Indonesian authorities" and that its security agencies had been working with Indonesian authorities.

"In response to this threat and the prevailing security situation, police and other agencies have been stepping up inland and border security measures," it said.

Indonesia, the world's most populous Muslim-majority country, has long struggled with 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 Daish 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 militants.

In January Daish-linked militants launched a deadly gun and bomb attack in Jakarta.