The police on Thursday disclosed that 669 cases of suspected money laundering had been reported by cryptocurrency exchanges between April and December.

The tipoffs came in the wake of a legal revision last April to prevent the transfer of criminal proceeds that obliges exchanges to report suspicious transactions believed to involve money laundering.

Although the National Police Agency has not disclosed what prompted the exchange operators to suspect wrongdoing, questionable transactions repeated frequently over short spans of time are believed to be among the main reasons.

Cryptocurrencies such as bitcoin, ethereum and ripple allow quick, easy and anonymous transactions to be carried out. They have been used to conduct illegal drug deals and pay for child pornography downloads.

In Japan, 16 cryptocurrency exchanges are registered based on the revised law on payment services. But ensuring security has become a major challenge.

In January, ¥58 billion ($540 million) worth of NEM virtual currency was stolen from the exchange Coincheck Inc., which had yet to have its application for registration approved.

In the meantime, the overall number of suspected money-laundering cases, including those reported by financial institutions, stood at 400,043 cases in 2017, down 1,048 cases from a year earlier, the police said.

Most of the reported incidents came from banks and other financial institutions, totaling 346,595 cases, followed by credit card companies at 15,448 cases and credit unions at 13,259 cases.