The United States warned citizens against travelling to Hong Kong, citing the risk of children being separated from parents as the Chinese city imposes controversial COVID isolation policies.

The State Department upgraded Hong Kong to its highest "Do Not Travel" warning "due to COVID-19 related restrictions, including the risk of parents and children being separated".

"In some cases, children in Hong Kong who test positive have been separated from their parents and kept in isolation until they meet local hospital discharge requirements," the State Department added.

The Asian financial hub is in the grip of its worst coronavirus outbreak, registering tens of thousands of new cases each day, overwhelming hospitals and shattering the city's zero-COVID strategy.

China has ordered local officials to stamp out the outbreak even as studies suggest as many as a quarter of the city's residents may have been infected in the current wave.

Authorities plan to test all 7.4 million residents later this month and are scrambling to build a network of isolation camps and temporary hospitals, with China's help, to house the infected.

"It remains our policy objective to subject all confirmed people to isolation at places other than their places of accommodation so as not to infect others," city leader Carrie Lam wrote in a progress report this week.

That has deepened anxieties about family separations in the months ahead and the warning by the United States is the first time the risk has been specifically cited in a travel advisory.

Spiralling infections

More than 220,000 infections have been recorded in the last two months, compared with just 12,000 for the rest of the pandemic - while its death rate is currently four times Singapore's.

The real figure is believed to be far higher in part because residents are worried about informing authorities they are infected.

For two years Hong Kong kept infections largely at bay using a strict zero-COVID strategy but an outbreak of the highly infectious Omicron variant has been tearing through the city since January.

The government was caught flat-footed with few plans in place to deal with a mass outbreak despite the two-year breathing room afforded by the zero-COVID success.

The city has since seen overflowing hospitals and morgues, shortages of medics and ambulances, panic buying and a frantic expansion of the city's spartan quarantine camp system.

The vast majority of those dying are over 70 and unvaccinated after Hong Kong failed to raise its elderly vaccination rate despite ample supplies.

The outbreak has led to the imposition of the toughest restrictions yet with more than a dozen types of businesses ordered to close and a ban on more than two people gathering in public. 

UK, Australia fears

Departures by foreign residents have spiked while businesses have voiced growing frustration and alarm over the city's descent into further international isolation. 

Last week it emerged some parents were being separated from children - including babies - who had tested positive and were being treated in hospital.

Hong Kong health authorities have defended the policy of separating sick children from their uninfected parents, saying that rapidly filling hospital spaces should be reserved for patients.

Last week, British consul general Brian Davidson said his team had "robustly challenged" the Hong Kong government over the practice, while the Australian consulate said it had also taken up the issue.

The United States, Britain and Australia are on a list of nine nations currently forbidden from flying to Hong Kong because of their own coronavirus infections. The ban was recently extended until late April.

Details are currently scant on how this month's mass testing will work and where the infected will be housed.

About 70,000 isolation units for mild cases are due to come online in the coming weeks, in requisitioned hotels, public housing units and camps.

At Hong Kong's current official caseload, that would cover roughly two days' worth of new infections.

Media reports citing government officials that some sort of lockdown will be ordered during the testing period have also fuelled panic buying this week.