David Bradbury, chief security officer of the identity management company Okta, said five of the company's clients had fallen victim to hacking groups known as ALPHV and Scattered Spider since August.

The stakes could not have been higher for the six companies, who depend on Macau's gambling industry for their profitability but have been bleeding billions of dollars due to China's strict Covid-19 rules.

The Chinese territory had been observing Covid-19 safety measures without any strict lockdowns until it found an imported coronavirus case on Oct 26.

While casinos will reopen, there will likely be no business for several weeks, executives and analysts say, with many Covid rules set to remain in place.

Casinos were last closed in Macau in February 2020 for 15 days in an unprecedented move to fight the virus.

Casinos are allowed to remain open while theatres, fitness centres, and leisure facilities must halt operations, chief executive Ho Iat Seng says.