BEIJING (Reuters) – Several districts of the Chinese capital set up security checkpoints, closed schools and ordered people to be tested for the novel coronavirus on Monday after an unexpected spike of cases linked to the biggest wholesale food market in Asia.

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

The return of the coronavirus has shrouded Beijing, home to the headquarters of many big corporations, in uncertainty at a time when China is trying to shake off the economic torpor caused by the disease.

“The risk of the epidemic spreading is very high, so we should take resolute and decisive measures,” Xu Hejiang, spokesman for the Beijing city government, told a news conference.

The outbreak has been traced to the sprawling Xinfadi market, 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 the city of Wuhan where the outbreak was first identified.

The new cases have led to officials in many parts of Beijing reimposing tough measures to stifle the spread of the virus, with at least three districts entering “war-time mode”.

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

Some districts sent officials to residential compounds in what they described as a “knock, knock” operation to identify people who had visited Xinfadi or been in contact with people who had.

Beijing began mass testing on Sunday, with tens of thousands of checks.

Samples of 8,950 people identified as recently being at Xinfadi had been collected as of early Monday, Gao Xiaojun, from the Beijing public health commission, told the briefing.

Results from the first 6,075 were negative, he said.

A vegetable wholesaler at Xinfadi said he had to be quarantined for 14 days at a hotel even though his test was negative.

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

FROM EUROPE?

The World Health Organization said on Sunday it was informed of the outbreak and an 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.

Foreign ministry spokesman Zhao Lijian, when asked whether China had shared data with the WHO about the cluster, told reporters he was not aware of the specifics but China and the WHO were in close communication.

Governments in other parts of China warned residents against non-essential travel to the capital, and implemented isolation protocols for some visitors from Beijing.

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

The northeastern province of Liaoning and Hebei in the north reported a handful of cases linked to the Beijing cluster. Sichuan in the southwest reported one suspected case.

Baoding, an industrialised city southwest of Beijing, was closely monitoring people arriving.

“Every gate to Baoding should be strictly guarded to prevent the contagion from spreading,” state media quoted Baoding officials as saying.