Image copyright Getty Images Image caption Gambling is illegal in China, except for Macau

Australian officials say they are investigating reports that 18 employees of Australian-based gambling firm Crown Resorts have been detained in China.

The employees are thought to include local staff and three Australians.

Australian media report that a "co-ordinated police operation" targeted marketing activities of Crown, owned by billionaire James Packer.

The people held are reportedly from the sales and marketing division and include a senior executive.

The 15 non-Australian employees detained are understood to be local employees at Chinese cities including Beijing and Shanghai.

Casino gambling is illegal in China, except in the former Portuguese colony of Macau.

Crown Resorts has interests there. For years it has skirted the question of how to advertise these gambling resorts in the rest of China by merely advertising the areas they are in.

Image caption Macau has been a gambling centre for hundreds of years

"Crown believes that a number of our employees in China are being questioned by local authorities," a company spokeswoman said. "At this time we can provide no further details."

It is not known whether any of the 18 arrested people have been charged with anything.

Australia's Department of Foreign Affairs and Trade issued a statement saying: "The Australian Government is aware of reports of the possible detention of a number of Crown International Group employees across China, including three Australians.

"Consular officials are seeking to confirm these reports with the relevant Chinese authorities."

A corruption crackdown initiated by Chinese President Xi Jinping has made many people, particularly government officials, wary of being seen with too much cash, and revenue in Macau's casinos fell by more than a third last year.