BEIJING (Reuters) – Several districts in Beijing reinstated security checkpoints, ordered residents be tested and closed schools on Monday in response to an unexpected resurgence of the coronavirus in the Chinese capital.

After nearly two months with no new infections, Beijing officials have reported 79 cases over the past four days, the biggest concentration of infections since February.

The return of COVID-19 in Beijing has shrouded the city, home to the headquarters of many big corporations, in uncertainty at a time China is trying to shake off the economic torpor of the virus.

“The risk of the epidemic spreading is very high, so we should take resolute and decisive measures,” Xu Hejiang, spokesman at the Beijing city government, said at a press conference on Monday.

The outbreak has been traced to the city’s Xinfadi, the biggest wholesale food market in Asia, where thousands of tonnes of vegetables, fruits and meats change hands each day.

A complex of warehouses and trading halls spanning an area the size of nearly 160 soccer pitches, Xinfadi is more than 20 times larger than the seafood market in Wuhan where the outbreak was first identified.

The spate of new cases prompted officials in many parts of the city to swiftly bring back tough counter-epidemic measures, with at least three districts entering “war-time mode.”

Measures imposed included erecting round-the-clock security checkpoints, closing schools and sports venues, and reinstating temperature checks at malls, supermarkets and office buildings.

Some districts also dispatched officials to residential compounds in what they described as a “knock, knock” operation to identify people who have visited Xinfadi or been in contact with somebody who has.

Beijing began testing on masse on Sunday, conducting tens of thousands of examinations.

Samples of 8,950 people who were identified as recently being at Xinfadi had been collected as of early Monday, said Gao Xiaojun, a spokesperson for the Beijing public health commission, at Monday’s briefing. Results from the 6,075 tested so far were negative, Gao said.

A vegetable wholesaler at Xinfadi said he had to stay in quarantine for 14 days at a designated hotel even after his test was negative.

“I’ve to take another test after the 14 days,” he said, speaking on condition of anonymity.

The World Health Organization said on Sunday it was informed of the outbreak and a subsequent investigation by Chinese officials.

“WHO understands that genetic sequences will be released as soon as possible once further laboratory analyses are completed,” it said in a statement.

An epidemiologist with the Beijing government said on Sunday a DNA sequencing of the virus showed the Xinfadi outbreak could have come from Europe.

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Governments in some other cities and provinces warned their residents against non-essential travel to the capital, and implemented isolation protocols for some visitors from the capital.

Wang Xiaoyang, who works in public relations in the southern boomtown of Shenzhen, said she received a text message from local authorities telling her to stay at home for 14 days after returning from Beijing on Friday.

The northeastern Liaoning province and northern Hebei province reported a combined handful of cases connected to the Beijing infections. Sichuan in the southwest reported one suspected case.

Baoding, a highly industrialised city in Hebei about 150 kilometres southwest of Beijing, was closely monitoring entry. “Every gate to Baoding should be strictly guarded to prevent the contagion from spreading in the city,” state media quoted Baoding officials as saying.