Beijing — Five million residents of a central Chinese city were confined to their homes Tuesday while another megacity shuttered all non-essential businesses, as the country battles a spate of coronavirus outbreaks including from the Omicron variant. Beijing is on high alert as it prepares to host the Winter Olympics next month, sticking to a zero-COVID strategy of targeted lockdowns, border restrictions and lengthy quarantines to try and eliminate the disease.

While those measures have kept the number of new cases far lower than virus hotspots in the United States and Europe, China is currently facing local flare-ups in multiple cities.

Flights cancelled, U.S. nationals warned

With more Chinese cities falling under harsh lockdown measures, and amid reports from some shuttered communities of shortages of essentials like food and medicine, the U.S. Embassy in Beijing sent an alert to Americans in the country warning that they could fall under restrictions with little warning.

The “measures may include mass testing, closures, transportation disruptions, lockdowns, and possible family separation,” the embassy said in the message, noting that it was “aware of reports of food and medical shortages in some affected areas.”

The embassy urged U.S. nationals in China to pay close attention to local media and government websites about restrictions in their areas, and warned that any “failure to follow the instructions of your local authorities may further complicate problems U.S. Citizens may encounter.”

Some of the coronavirus case in China have been in travelers arriving from the United States, prompting the Chinese aviation regulator to order more than 60 scheduled flights from the U.S. to be canceled in recent weeks, according to the Reuters news agency.

Omicron challenges zero-COVID plan

The fast-spreading Omicron variant has presented a fresh challenge for China, with two cases in Anyang — the newly locked-down city in Henan province — linked to a growing infection cluster in the northern metropolis of Tianjin, some 250 miles away. Authorities in Anyang announced the lockdown late Monday, issuing a notice ordering residents not to leave their homes or drive their cars, state news agency Xinhua reported.

All non-essential businesses have been closed and a mass-testing drive has been launched “to respond to the severe epidemic control situation and strictly prevent the spread of the Omicron virus outbreak,” Xinhua reported.

There were 58 new local infections reported in Anyang, state broadcaster CCTV said Tuesday, bringing the city’s total caseload to 84 since Saturday.

The city had already restricted outbound travel, which local officials said was to “ensure that the outbreak does not spill over into outside areas.”

At least three cities in Henan are battling emerging outbreaks, with provincial capital Zhengzhou’s 13 million inhabitants inching closer to a full lockdown. Under current rules, schools and kindergartens have been closed along with all non-essential businesses.

Last week, one million people in the city of Yuzhou were put under stay-at-home orders.

Elsewhere, Tianjin — a major port city just 93 miles from Beijing — has barred people from leaving without official permission, ordered the testing of all 14 million residents, and cancelled trains into the capital.

Tianjin, which is adjacent to Winter Olympics venues in Beijing and Hebei province, confirmed another 10 new locally transmitted cases after citywide mass testing.

Zhang Boli, of the Chinese Academy of Engineering, told local television that the Tianjin cluster is the first time Omicron has been detected spreading within China.

The northern city of Xi’an is in its third week of lockdown as it attempts to stamp out a 2,000-case outbreak.

There are also flare-ups in southern China.

The Chinese territory of Hong Kong is battling its own Omicron outbreak that began with returning airline crew.

The outbreak has upended the city’s zero-COVID success and seen a reintroduction of strict social-distancing measures including the closure of bars and gyms, and an end to evening restaurant dining.

On Tuesday, chief executive Carrie Lam announced further measures including the closure of all kindergartens and primary schools until after the Lunar New Year in early February.

Her administration has hewn to China’s closed-off blueprint but is facing growing anger from both residents and international businesses over the lack of an exit strategy.

Across the border from Hong Kong, the mainland tech hub of Shenzhen has also logged a handful of cases in recent days, sparking lockdowns of some housing compounds, a mass-testing initiative and travel restrictions.

The city’s Yantian port is one of the world’s busiest, and there are fears that further lockdowns or restrictions there and in Hong Kong could compound already acute global supply problems.