Construction crews from mainland China were helping Hong Kong build two temporary isolation facilities to house thousands of coronavirus patients on Sunday as a senior official declared the city "in full combat mode".

The crowded Chinese financial hub is in the throes of its worst-ever coronavirus outbreak, registering thousands of confirmed cases a day as hospitals reach breaking point.

A strict zero-Covid policy like China uses kept infections at bay for two years but left the city cut off internationally.

And when the highly transmissible Omicron variant broke through, authorities were caught flat-footed with a dangerously under-vaccinated elderly population and few plans in place to deal with a mass outbreak.

Late Saturday city leader Carrie Lam announced that China State Construction International Holdings, the largest state-owned constructor in Hong Kong, would start work on two temporary isolation facilities to provide 9,500 extra beds.

The units will be located at Penny's Bay, which already hosts a quarantine camp, and in Kai Tak where the city's old airport once stood.

Lam also announced that three hotels would be used to create an additional 20,000 beds.

Chief Secretary John Lee, Hong Kong's number two official, wrote on his official blog on Sunday that the city's government was in "full combat mode".

"With our motherland's strong support, we will definitely win the battle," Lee wrote.

The sudden flurry of activity came after Chinese President Xi Jinping ordered Hong Kong to make tackling the outbreak its "overriding mission" in comments that were seen as something of a rebuke to the city's leadership.

It is not yet clear when the new facilities will be ready and whether they will be enough given Hong Kong's spiralling caseload.

Under China's direction, Hong Kong is sticking to a policy of trying to isolate everyone who tests positive for the coronavirus and has rejected calls to shift to a strategy of living with Covid.

Over the last few days officials have announced around 6,000 confirmed cases daily with a similar number of "preliminary positives" that still need to be certified.

About 22,000 cases have been recorded since the current outbreak hit last month compared to just 12,000 in the two years before that.

Some hospitals have had to house patients on gurneys outdoors in grim winter conditions while thousands are still waiting at home in the city's notoriously small apartments with positive test results.

Ben Cowling, a coronavirus expert at the University of Hong Kong, said isolation facilities would be useful but increasing hospital beds must be a priority.

"New cases needing admission will continue to accumulate faster than beds are freed up, and delays to admission will get longer and longer," Cowling wrote on Twitter.

"Construction of isolation facilities for mild/asymptomatic cases will be useful for people that can't isolate at home... but increasing hospital beds and ICU beds must be a priority."

Lam announced plans on Friday to test Hong Kong's entire 7.5 million population by some point in March, when modellers predict the daily caseload could reach 28,000.

She has ruled out the kind of hard lockdown that China has used to stamp out smaller outbreaks.