The detection of the first clusters of atypical pneumonia in Wuhan doesn't necessarily mean that COVID-19 crossed from animals to humans from there, a senior World Health Organization (WHO) expert said Monday.

Dr Michael Ryan, executive director of the WHO Health Emergencies Program, said this at a routine COVID-19 briefing, reports Xinhua.

A much more "extensive retrospective epidemiological study" should be carried out to fully recognise the links between the cases, he said.

The expert also laid emphasis on the need to start studies on the first reported human clusters in order to systematically look for the "first signal at which the animal-human species barrier was crossed," before moving to the studies on the animal side.

The WHO advance team that travelled to China in preparation for an international mission of identifying the zoonotic source of COVID-19 has concluded its mission recently, according to the WHO expert.

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Future studies will build on the initial investigations done by Chinese experts around the Wuhan seafood market.

Ryan noted that WHO is moving forward with agreeing on the international team and ensuring that right expertise will be in place to work with the Chinese counterparts to design and implement further studies.