GENEVA (Reuters) – The origins of a new cluster of coronavirus infections in Beijing are uncertain, World Health Organization officials said on Monday, describing the claim that it might have been caused by imports or packaging of salmon as a “hypothesis”.

Several districts of the Chinese capital put up security checkpoints, closed schools and ordered people to be tested for the coronavirus on Monday after an unexpected rise in cases linked to the biggest wholesale food market in Asia.

State-run newspapers reported that the virus was discovered on chopping boards used for imported salmon at Beijing’s Xinfadi market amid worries about a second wave of the pandemic in China.

Mike Ryan, head of the WHO’s emergencies program, said in a briefing that he would be “reticent” to say that packaging needs to be tested as a result of the new infections.

His comments echoed those of experts earlier on Monday who said the fish itself was unlikely to carry the disease and any link to salmon may have been the result of cross-contamination.

Ryan said the U.N. agency was closely tracking the outbreak, which is worrying given its appearance in a major city like Beijing, and is in close contact with Chinese authorities as they seek to control it.

“As we have seen in many countries, the emergence of new clusters … is always a concern,” he said.

“But what we do like to see is an immediate response to that and comprehensive set of measures.”

There had been more than 100 cases confirmed in the new outbreak but no reported deaths, WHO officials said.

Ryan said he “fully expects” China to publish the genetic sequencing of the virus from the Beijing outbreak when it is ready. Genetic traces have suggested it could have come from Europe.

“The finding that this may represent a strain more common in transmission in Europe is important and it may reflect human-to-human transmission more than any other hypotheses,” he said.

“But that remains to be seen.”