Investigators on Tuesday said they were still trying to find the animal vector through which the COVID-19 virus was transmitted to humans.

Scientists think the disease originated in bats, but the World Health Organization (WHO) team visiting the Chinese city of Wuhan is looking into how it could have made the jump to humans via another mammal.

The WHO presented its findings so far alongside Chinese experts at a joint study news conference.

What has the WHO found so far?

WHO virus expert Peter Ben Embarek told the briefing that work to identify the origins of the coronavirus pointed to a natural reservoir in bats.

However, he said the absence of bat environments in Wuhan indicated that it had been transmitted to humans through another animal.

Widespread testing for the virus had been carried out across animal species in many Chinese provinces, but this had failed to yield a result.

"Through these studies, it has not been possible to pinpoint any animal species as a potential reservoir for this disease," Embarek said.

The possibility remains that the virus was directly transmitted to humans from bats, or that it had somehow been transmitted through food — particularly frozen products.

The team remains hopeful that the animal source can still be identified.

Team member Marion Koopmans said it was important to trace back all animal species and products that were on the market. This had already shown that there were some animals confirmed as susceptible, like rabbits, or suspected of being susceptible, such as ferrets, badgers or bamboo rats.

Lab leak eliminated as explanation

The WHO expert essentially ruled out a fourth theory that the novel coronavirus had somehow leaked from a Chinese lab.

"The laboratory incident hypothesis is extremely unlikely to explain the introduction of the virus into the human population," said Embarek. "Therefore it is not in the hypothesis that we will suggest for future studies."

He agreed with a Chinese assessment that there was "no indication" the disease was circulating in the city before December 2019, when the first official cases were recorded.

Why is the visit only taking place now?

The team from 10 different nations has visited hospitals, research institutes, the traditional wet market tied to the outbreak and other sites.

Their visit took months to negotiate after China only agreed to it amid massive international pressure. Beijing has continued to refuse appeals for a strictly independent investigation.

Thorny diplomatic issues lie behind the WHO visit, with Washington demanding a "robust" probe and Beijing issuing a warning not to "politicize" the investigation.

Authorities in China have tightly controlled information about the possible causes of the pandemic that has now infected more than 106 million people and killed more than 2.3 million.