HONG KONG (AFP) — Loan sharks earn tens of millions of dollars a year by providing underground money exchange services and cash smuggling networks for Chinese gamblers in Macau, China’s Public Security Ministry said Friday.

Macau, known as the Las Vegas of the East, is the only city in China where gambling is legal, but the mainland Chinese visitors who fuel the city’s multibillion-dollar economic pillar are bound by the country’s strict foreign exchange controls.

Under those restrictions, a Chinese citizen is allowed to change no more than $50,000 worth of yuan into foreign currency in a given year, and to carry no more than $5,000 in cash in or out of the country on any one trip.

But Chen Shiqu, the deputy crime bureau chief of Beijing’s Public Security Ministry, said Friday there had been a surge in recent years of illegal “money changing parties,” state media reported.

These parties changed Chinese yuan into Hong Kong dollars in underground banks in Zhuhai — a mainland city adjacent to Macau — and hired a stream of individuals to smuggle the cash across the border to gamblers “like ants moving home,” Chen said.

“Each year, the money masters behind the scenes make profits of more than 1.5 billion Hong Kong dollars ($192 million), and the profiteering drive has led to the rapid expansion of the groups,” he added.

In one case, a syndicate of more than 30 people recruited more than 100 cash smugglers on the Chinese video platform Douyin by posing as a hiring agency.