Authorities in Beijing have described the city’s Coronavirus outbreak as “extremely severe” as dozens more cases emerged and travel from the city was curtailed. Additional neighbourhoods were fenced off on Tuesday, with security checkpoints set up at residential compounds, and high-risk people – such as close contacts of people who test positive – were prevented from leaving the city.

“The epidemic situation in the capital is extremely severe,” Beijing city spokesman Xu Hejian warned at a press conference. “Right now, we must take strict measures to stop the spread of Covid-19.” More than 20 neighbourhoods in the Chinese capital have now been designated medium risk, which means authorities can impose stricter restrictions on the movement of people and cars and can carry out temperature checks. Health authorities said sealed-off residences and people in quarantine would have food and medicine delivered to them.

The outbreak is the most significant in China since February, prompting fears of a second wave and questions over how the virus was able to spread given severe quarantine measures taken by authorities. The Coronavirus outbreak is potentially embarrassing for Beijing, which had declared victory over the virus and ordered citizens back to work. The capital, where measures were among the strictest in the country, had reported no new locally transmitted cases for 56 consecutive days before a cluster of diagnoses began on Thursday. Before that most new cases had come from Chinese nationals returning from abroad.

On Tuesday, state media appeared to push the idea that the virus had come from abroad. Wu Zunyou, chief epidemiologist at the Chinese Centre for Disease Control and Prevention, said the strain was most similar to those seen in Europe, the US or Russia. “It clearly indicates the virus strain is different from what it was two months ago,” he told state broadcaster CGTN. “The virus strain is the major epidemic strain in European countries. So it is from outside China brought to Beijing.”

Similarly, a deputy director at the pathogen biology department at Wuhan University told state media he believed the new outbreak involved a more contagious strain of the virus than the one that hit Wuhan at the beginning of the pandemic. European scientists said it was too early to tell whether the virus causing the new Beijing Coronavirus outbreak differed in transmissibility from the original.

“Genetic sequencing could easily be established if it was imported from Europe. But with the limited information currently available, “the most that can be interpreted – if indeed it does look like being from Europe – is that it is good evidence of control of the original Chinese spread and no resurgence of that virus,” said Deenan Pillay, professor of virology at University College London. The outbreak – linked to 106 cases, including 27 reported on Tuesday – has been traced to the Xinfadi wholesale food market in south-west Beijing’s Fengtai district, which sells thousands of tonnes of food daily and has been visited by more than 200,000 people since 30 May.

Samples of the virus were discovered on chopping boards used for imported salmon at the market, fuelling speculation it had come from abroad even though experts have said fish is unlikely to carry the disease and any link to salmon may have been the result of cross-contamination. The World Health Organization’s emergencies director, Mike Ryan, said he expected Chinese authorities to publish the genetic sequencing of the Coronavirus outbreak in Beijing and supported their efforts so far. He said the idea that imported salmon caused a new outbreak was not the “primary hypothesis”.

“A cluster like this is a concern, and it needs to be investigated and controlled – and that is exactly what the Chinese authorities are doing,” he said. On Tuesday, Zhao Lijian, spokesman for China’s foreign ministry, said Beijing had asked Canada to investigate parasites found in fish shipments. Norwegian exporters have said China has halted salmon imports.

After months of economic paralysis, authorities have tried to limit lockdown measures to parts of the city and encouraged residents to continue “life as normal” while taking extra precautions. Provinces as far away as Yunnan in the south have brought rules requiring quarantine for people returning from Beijing. Shanghai authorities announced that all arrivals from medium– and high-risk areas must undergo 14 days of quarantine. Some long-distance bus routes connecting Beijing and other provinces were suspended.

All indoor sports and entertainment venues in the city were closed on Monday. Coaches and players from the Beijing Super League football team, Guoan, have been tested and given a week off because their training camp was in the same district as the outbreak source, local media reported. In Xicheng district, which borders Fengtai, another market was shut after a coronavirus case was confirmed. Seven neighbourhoods near the market have been closed off.

More than 8,000 workers from the market have now been tested and sent to centralised quarantine facilities, and other Beijing wet markets, basement markets and more than 30,000 restaurants are being disinfected. Health authorities have entered what state media termed “wartime mode” in response.

More than 76,000 nearby residents were tested on Sunday across almost 300 testing points, authorities said, and strict measures have been put in place, including school closures and transport suspensions, ride-sharing, and taxi services. Since the first virus case was detected last year in Wuhan, China has reported more than 84,000 cases, and more than 4,600 people have died.

Chinese officials and state media have quickly defended the country amid fears of a second wave. “Control measures have been in place in communities. Three officials accountable were dismissed,” said the editor-in-chief of the Global Times, Hu Xijin. “US politicians will likely see a miracle that Beijing can have zero new cases in a month.”

Three people have been dismissed over the outbreak, including the head of the Xinfadi market, the local Communist party chief, and the district deputy head.

Additional reporting by Lillian Yang, Sarah Boseley and agencies