China reported Monday its highest number of daily coronavirus cases in about two years as clusters emerged in more than a dozen cities, posing a fresh challenge to Beijing's "zero-COVID" stance.

The country's borders remain mostly closed as policymakers continue to pursue the zero-tolerance approach even as many parts of the world turn to living with the virus.

More than 500 infections were reported across mainland China on Monday, the most since China's initial outbreak in the central city of Wuhan was brought under control in the middle of 2020.

The spike comes as cases spiral out of control across the border in the southern Chinese territory of Hong Kong, where hospitals have been overflowing with patients and locals are panic-buying fearing a lockdown.

Questions have been raised about the sustainability of China's heavy-handed control strategy coupled with concerns about the efficacy of Chinese vaccines.

COVID-19 was first detected in China in late 2019 and since then Beijing has responded to each local outbreak with harsh snap lockdowns and mass testing along with state-mandated tech to track people's movements.

In Hong Kong, there have been mixed messages about whether officials will follow the mainland's lockdown policy and there have also been signs that Beijing is reconsidering its zero COVID-19 policy. Hong Kong reported 25,150 new coronavirus infections and 280 deaths on Monday, as authorities struggle to contain a worsening COVID-19 outbreak that has torn through hundreds of nursing homes and hit many of the city's unvaccinated elderly. Health authorities said 161 of the deaths reported on Monday were in the past 24 hours while 119 were older fatalities processed with a delay.

Hong Kong reported the most deaths globally per million people in the week to March 6, according to data publication Our World in Data. The figures come as the global financial hub clings to a "dynamic zero" coronavirus strategy, which as in mainland China seeks to eradicate all outbreaks. Many other countries are meanwhile shifting to a strategy of co-existence with the virus.

The deluge of cases has swamped Hong Kong hospitals, isolation centers and funeral parlors beyond capacity and left the health care system, public transport, malls, postal services, supermarkets and pharmacies struggling without staff. Anxious residents have emptied supermarkets ahead of expected compulsory mass COVID-19 tests, with many shelves left bare for over a week. Many businesses including restaurants and stores have shuttered, with main districts notably quiet and few residents outside on the streets. The city has implemented its most stringent restrictions since the pandemic started in 2020, with a ban on public gatherings of more than two people and most venues closed. Flights into the city are banned from most countries, including the United States and Britain.

A top Chinese scientist said last week that the country should aim to co-exist with the virus and could move away from the zero-tolerance strategy "in the near future." However, National People's Congress spokesperson Zhang Yesui poured cold water on that idea Friday ahead of China's annual parliamentary meetings where policies are set for the coming year.

"The path is correct and results are good. Any prevention and control measures will have some costs, but compared to protecting people's lives and health, these costs are worth it," Zhang said.

China had reported 214 domestically transmitted cases with confirmed symptoms for Sunday, the majority in the provinces of Guangdong, Jilin and Shandong. It's the highest daily caseload since early March 2020 when authorities began to count locally found infections and cases arriving from outside the mainland separately.

China's latest local virus flare-ups are tiny by global standards and the country is sticking to its "dynamic-clearing" policy which requires local authorities to quickly identify and quarantine every infection and their close contacts as well as impose travel restrictions.

The number of locally transmitted asymptomatic cases rose to 312 for Sunday, the highest daily number since China started in late March 2020 to classify symptomless infections separately from confirmed cases.

In Shanghai, although only three local symptomatic cases were reported on Sunday, local asymptomatic cases hit a near two-year high of 45.

The city's mass vaccination program has led to a larger number of asymptomatic cases, a Shanghai health official said at a news conference on Sunday.

All but one of the 48 Shanghai cases were found among people already under quarantine. Shanghai has tested more than 100,000 people and temporarily closed some residential compounds in a bid to contain outbreaks.

There were no new deaths on Sunday, leaving the death toll unchanged at 4,636. As of March 6, China had reported 111,195 cases with confirmed symptoms, including both local ones and those arriving from outside the mainland.