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Three Australian employees of gambling group Crown Resorts, who were detained in China last month, have been formally arrested.

The trio were among 18 Crown staff held after a police operation, believed to target Crown's marketing activities.

They include Jason O'Connor, a senior executive in charge of attracting Chinese high-rollers to Australia.

Casino gambling, and promoting gambling abroad, are illegal in mainland China.

Foreign Minister Julie Bishop said the three were arrested on suspicion of gambling offences.

Australian officials have met the men to offer advice and support, as well as passing on messages from their families.

"There is an anti-corruption agenda of President Xi Jinping and gambling is one of the areas of interest, so we are subject to the laws of China," the minister said.

A Chinese foreign ministry spokesman said Beijing would "handle this case in accordance with the law".

Of the 18 people originally arrested, one has been bailed, but the fate of the remaining 14 people - all believed to be Chinese nationals - is unclear.

Crown Resorts, controlled by Australian billionaire James Packer, is yet to comment on the latest development.

But like other casino groups across Asia, it sees wealthy Chinese gamblers as an important part of their business.

International high-rollers, known as VIPs, gambled $46.8bn (A$61bn) in the last financial year in Crown's Melbourne and Perth casinos.

Crown said that Chinese gamblers only made up half this total, and counted for just 12% of total revenue for the business.

Casinos in Australia were seen as one potential beneficiary from a corruption crackdown initiated by Chinese President Xi Jinping - which pushed gamblers away from the casinos of Macau - where revenue fell by more than a third last year - to places further afield.