Interpol has launched an investigation into an alleged "baby factory" after it emerged that a Japanese businessman had fathered 16 surrogate children and expressed a desire for many more.

Police raided a home in Bangkok earlier this month, finding nine babies with nine nannies. Mitsutoki Shigeta, 24, has been identified as the father of each of them, as well as seven more. Authorities are looking into his motives, which he says are benign.

The children were apparently housed in unfurnished rooms filled with baby bottles, bouncy chairs, playpens and nappies, according to Thai police.

"What I can tell you so far is that I've never seen a case like this," Thailand's Interpol director, police Maj Gen Apichart Suribunya, said on Friday.

According to the Associated Press (AP), he added: "We are trying to understand what kind of person makes this many babies." The investigation will centre on Japan, Cambodia, Hong Kong and India.

"We are looking into two motives. One is human trafficking and the other is exploitation of children," said police Lit Gen Kokiat Wongvorachart, Thailand's lead investigator in the case.

He said Shigeta had made 41 trips to Thailand since 2010. On many occasions he travelled to nearby Cambodia, where he brought four of his babies.

The founder of the New Life clinic, a multinational fertility clinic that provided Shigeta with two surrogate mothers, said she warned Interpol about him even before the first baby was born in June 2013, the agency reported.

"As soon as they got pregnant, he requested more. He said he wanted 10 to 15 babies a year, and that he wanted to continue the baby-making process until he's dead," said Mariam Kukunashvili.

She told the AP, Shigeta also asked about equipment to freeze his sperm to have sufficient supply when he was older.

And she said he told the clinic's manager that "he wanted to win elections and could use his big family for voting", and that "the best thing I can do for the world is to leave many children".

Kukunashvili, who is based at the company's headquarters in the country of Georgia, said she never met Shigeta but received reports from her Thai staff. The clinic is based in Thailand and six other countries.

She said she also emailed Shigeta, whose then lawyer responded to say he was involved in "no dishonesty, no illegal activities". He has not been charged with any crime and, according to reports, is trying to get his children back.

The 12 in Thailand are being cared for by social services and he has proven through DNA samples sent from Japan that he is their biological father.

He quickly left Thailand after the raid on his condominium on 5 August and has said through a lawyer that he simply wanted a large family and has the means to support it, AP reported.

Thai police said Shigeta hired 11 Thai surrogate mothers to carry his children, including four sets of twins. Police have not determined the biological mothers, Kokiat said.

The case came to light after a tip-off that followed allegations that an Australian couple abandoned a surrogate baby after learning it had Down's syndrome. The couple deny the accusation.

During the raid on the property, Shigeta's former lawyer Ratpratan Tulatorn insisted his client had done nothing wrong.

"These are legal babies, they all have birth certificates," Ratpratan told Thailand's Channel 3 television station. "There are assets purchased under these babies' names. There are savings accounts for these babies, and investments. If he were to sell these babies, why would he give them these benefits?"