HONG KONG, July 18 (Reuters) - Macau kicked off an 11th round of COVID-19 testing for residents on Monday, as the world's biggest casino hub extended a lockdown of casinos and other businesses in the fight on its worst outbreak since the pandemic began.

Despite a steady fall in infections that saw just 27 new cases reported on Sunday, authorities said the city's more than 600,000 residents must stay home, with all non-essential businesses shut, until Friday.

Although more than 90% of residents are fully vaccinated against COVID, this is the first time Macau has had to grapple with the fast-spreading Omicron variant.

The Chinese special administrative region, which went into lockdown on July 11, has a tally of more than 1,700 infections since the middle of June, after having been largely COVID-free following an outbreak in October 2021.

The former Portuguese colony has only one public hospital and its medical system was already stretched before the outbreak.

Macau follows China's "zero-COVID" policy of stamping out all outbreaks at just about any cost, which runs counter to a global trend of trying to co-exist with the virus.

Coronavirus testing for all will be done at least twice this week, with rapid antigen tests also required in between.

The six casino operators, Sands China, Wynn Macau , MGM China, Melco Resorts, Galaxy Entertainment, SJM Holding, are racking up losses ahead of bids for new licenses by next month.

Still, daily infections are fewer than elsewhere, such as neighbouring Hong Kong, where cases have jumped this month to more than 3,000 a day as curbs have started to ease.

But Macau has an open border with mainland China, and many residents live and work in the adjoining city of Zhuhai.

(Reporting by Farah Master; Editing by Clarence Fernandez)