Indonesian police have arrested three suspects over a plan to sell a three-month-old baby boy to a buyer in Singapore for almost $11,000.

Acting on a tip-off, police from the Riau Islands province raided a house in Batam last Wednesday and arrested three Indonesians - the baby's father, a midwife and a broker.

Indonesian police identified them by their initials: BAH, aged 55; YA, 31; and EA, 47.

Police also seized a documented receipt confirming the transaction of a baby for US$8,000 (S$10,700).

"Investigations are ongoing but we have found strong evidence of baby-selling," provincial police spokesman Hartono told reporters yesterday.

The boy's parents, who are Chinese, were facing economic hardship, he said.

Superintendent Hartono would not disclose the buyer's identity.

Anti-trafficking groups say the case is only the tip of the iceberg. Mr Syamsul Rumangkang, head of the non-governmental Anti-Trafficking Agency in Batam, said the baby-selling racket is an "immaculate network" involving many people.

Some of the transactions take place even before the babies are born, he said. "So the pregnant woman goes to Singapore and gives birth there. The buyer, usually from the middle class, has prepared everything," he said.

Each baby can be sold for US$5,000 to US$10,000.

Trafficking of babies from Indonesia to other countries, including Singapore, is not new. A long-time human trafficker, a midwife and brokers were among seven women arrested in 2013 after police found a four-month-old baby, named Teddy Lukas, destined for Singapore at a house in Jakarta.

A Singaporean identified only as Mr Chow had bought Teddy and visited Jakarta to check his health before handing a down payment of $500 to the broker.