The Chinese capital has seen 100 new locally transmitted coronavirus cases, the WHO says.
The Chinese capital Beijing has put more neighbourhoods under lockdown and boosted testing as it tries to contain an outbreak of coronavirus.
There were 27 new cases reported on Tuesday, bringing the total to 106 people over five days.
A Chinese official has described the new outbreak in the capital as "extremely severe".
For more than seven weeks Beijing had only registered cases from people travelling in from abroad.
New clusters of coronavirus are "always a concern", said Mike Ryan, emergencies programme head at the World Health Organization.
"But what we do like to see is an immediate response to that and comprehensive set of measures," he added.
What do we know about the new cases?
The fresh outbreak has been linked to the city's largest wholesale market, Xinfadi.
Local media reports say the virus was discovered on chopping boards used for imported salmon at the market, prompting major supermarkets in Beijing to pull the fish from their shelves.
The general manager of the market has been dismissed, along with other local officials.
In his comments though, the WHO's Mike Ryan was cautious about the source of the outbreak, saying the suggestion it was carried on salmon or its packaging was only a "hypothesis".
Other health experts have pointed to cross-contamination as being a more likely cause.
Zeng Guang, chief epidemiologist of China's Centre for Disease Control and Prevention (CDC), said the virus strain found in Beijing did not resemble the type circulating across the rest of the country, and the WHO has urged China to share the genetic sequencing.
What's being done to combat the outbreak?
The market was shut early on Saturday and restrictions imposed on nearby neighbourhoods. Residents living near the market have been told they can't leave the city.
All taxis are also now banned from driving beyond the city limits.
By Monday Beijing had set up nearly 200 testing sites and contacted about 200,000 people who had visited the market since the end of May, state news outlet Xinhua reported.
Volunteers at security checkpoints tested residents' temperatures.
The lockdown has also been extended to 21 residential compounds close to the market.
Indoor sporting and entertainment venues across Beijing have been ordered to close, as have many schools.
The concerns over a second outbreak come as Beijing attempts to return to normal life. China reported the first coronavirus cases last year, but new cases and infections have dropped drastically as hotspots have developed elsewhere in the world. BBC