The World Health Organization has ruled out a laboratory accident as the cause of the coronavirus pandemic, saying it was “extremely unlikely” that the pathogen leaked from a Chinese facility.

A WHO scientific team has spent the past few weeks in Wuhan to learn more about the origins of the virus. It has also visited the Wuhan Institute of Virology, which has been the subject of unfounded allegations that the Sars-Cov-2 virus that caused Covid-19 may have leaked from its biosafety laboratory.

Peter Ben Embarek, head of the visiting WHO team, said it had appraised various hypotheses and concluded the evidence weighed against the virology institute or another of the Chinese city’s labs being the source.

While “accidents do happen”, virus leaks from high-level biosafety labs were “extremely rare” and the WIV set-up made it “very unlikely that an escape could happen from a place like that”, he said.

The most plausible explanation was the accepted theory that the virus originated in bats and jumped to an as yet unknown intermediate host species before infecting humans, he added.

Beijing’s careful management of the trip and a delay of more than a year between the virus being detected and the team’s arrival in Wuhan have raised doubts about the investigation’s ability to draw hard conclusions concerning the politically charged issue of the virus’s origin.

Ben Embarek acknowledged the field research conducted in Wuhan did not “change dramatically” the team’s broad understanding of what had happened.

That the WIV was known to be experimenting on coronaviruses before the pandemic began has stoked the lab leak theory. Mike Pompeo, former US secretary of state in Donald Trump’s administration, was one of those to advance the unproven allegation.

Liang Wannian, an expert from China’s National Health Commission who led a team of Chinese scientists working alongside the WHO, said that the inability to find samples of Sars-Cov-2 in Wuhan’s labs added to evidence against the leak theory.

Liang said studies to sample animals that might have been the intermediary host for the virus should extend beyond China, adding that tens of thousands of samples from wild and farm animals inside the country had already tested negative for the virus.

Liang also noted that infections were discovered in different parts of Wuhan before the first known cases linked to the Huanan seafood market, making it less likely to be the source of the outbreak.

The WHO team’s research focused primarily on the early stages of the virus outbreak in China and concluded that the evidence available from hospital records and blood samples did not suggest the virus had been circulating widely in Wuhan before December 2019, when the first cases were reported.

The team did not rule out the possibility of the virus arriving in Wuhan on frozen or refrigerated foods, a theory endorsed by a number of Chinese scientists.

Many in China have also pushed the idea that the virus was already circulating and infected humans elsewhere before the Wuhan outbreak.

Marion Koopmans, a virologist on the WHO team, said it was open to looking at evidence from countries outside China. The investigation intended to “go and search for evidence of earlier circulation wherever that is indicated”, she added.

A report on the WHO team’s findings is expected to be released in the coming weeks.