Hong Kong authorities said Sunday that one of 77 hamsters handed in by pet owners for a Covid cull has tested positive, as thousands of city residents remain under lockdown to combat an outbreak.

The financial services hub adheres to mainland China’s strict “zero-Covid” policy under which even the slightest sign of the virus is stamped out with contact tracing, targeted lockdowns and long quarantines.

More than 2,000 hamsters have been culled after some imported from the Netherlands by a local pet shop were found to be Covid-positive, raising fears of animal-to-human transmission.

The risk of transmission from animals “remains low”, the World Health Organization has said, but is a possibility.

It comes as Hong Kong battles an Omicron cluster that has infected more than 170 people in a single public housing estate in recent days.

Pet owners who bought a hamster after December 22 were urged last Tuesday to surrender their pets for testing and killing.

A range of small animals found in pet shops with the imported hamsters were seized and put down by authorities, including guinea pigs and rabbits.

But on Sunday authorities said among 77 hamsters surrendered only one was positive for the virus.

Hong Kong recorded its highest number of new Covid cases in 18 months on Sunday at 140, placing the city’s zero-tolerance strategy under pressure.

Nearly 5,000 residents at a high-rise public housing estate in the densely populated Kwai Chung neighbourhood have been placed under a five-day lockdown and all 35,000 people on the estate ordered to take Covid tests.

The cluster is one of the largest and fastest-growing outbreaks Hong Kong has seen yet, with 170 cases recorded in the development so far.

But the government’s order to surrender hamsters and its lockdown of the Kwai Chung estate have sparked questions about the viability of the zero-Covid policy.

Hong Kong leader Carrie Lam has defended the strategy, saying it is necessary to protect hospitals.

The city’s top microbiologist and government adviser on Covid told reporters Sunday that “zero-Covid” was not a goal but a way to “buy time for people to get vaccinated”.

Hong Kong has so far recorded more than 13,000 infections and over 200 Covid-related deaths, making it one of the world’s most successful places in keeping the virus at bay.