A new coronavirus has been discovered in the blood of five bats which had been living in a cave in Thailand.
Researchers say their findings suggest a "high probability of bat origin" of Covid-19, which is currently causing the global pandemic.
Their discovery extends the area in which related viruses have been found to a distance of 4,800km (2,983 miles).
This means related viruses could be present in animals in Japan, China and Thailand, the researchers said in a report published in Nature Communications.
In the paper, researchers say coronaviruses "with a high degree of genetic relatedness to Sars-CoV-2 are widely present in bats across many nations and regions in Asia".
But fortunately, the virus is not thought to be able to infect humans as it cannot bind to the ACE2 receptor on human cells, although Covid-19 can.
The authors say they believe coronaviruses only evolve the ability to infect human cells after first being passed into an intermediate host, such as a pangolin.
The isolated virus, named RacCS203, is a close match to the genetic code of SARS-CoV-2, exhibiting 91.5% similarity in their genomes, researchers say.
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It is also closely related to the RmYN02 isolate found in bats in Yunnan, China.
They say the antibodies were able to neutralise the pandemic virus, which is further evidence that SARS-CoV-2-related coronaviruses are circulating in Southeast Asia.
The paper's abstract says: "Whole genome sequences were obtained from five independent bats in a Thai cave yielding a single isolate (named RacCS203) which is most related to the RmYN02 isolate found in Rhinolophus malayanus (Malayan horshoe bats) in Yunnan, China.
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"SARS-CoV-2 neutralizing antibodies were also detected in bats of the same colony and in a pangolin at a wildlife checkpoint in Southern Thailand.
"Antisera raised against the receptor binding domain (RBD) of RmYN02 was able to cross-neutralize SARS-CoV-2 despite the fact that the RBD of RacCS203 or RmYN02 failed to bind ACE2.
"Although the origin of the virus remains unresolved, our study extended the geographic distribution of genetically diverse SC2r-CoVs from Japan and China to Thailand over a 4800-km range.
"Cross-border surveillance is urgently needed to find the immediate progenitor virus of SARS-CoV-2."
Professor Martin Hibberd, from the London School of Hygiene & Tropical Medicine, who was not involved with the study, said: "This paper has identified a further bat coronavirus that is thought to be related to the SARS-CoV-2 strain currently causing the human pandemic.
"This new finding from Thailand emphasises the broad distribution of the bats and viruses that may include the originator of the current outbreak."
Speaking on Tuesday, a team of World Health Organization investigators say their month-long investigation into Covid-19's origins in China shoes previously-made claims that the virus came from a lab are "extremely unlikely".
The WHO investigative team said that an animal intermediary is the most likely source of SARS-CoV-2 – although it is still unclear exactly which animal it came from.
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