Scientists claim to have found more clues about how the new coronavirus could have spread from bats through pangolins and into humans, as India reported its worst single-day rise in new cases, and the number of Covid-19 infections worldwide neared 6 million.

Writing in the journal Covid-19 Science Advances, researchers said an examination of the closest relative of the virus found that it was circulating in bats but lacked the protein needed to bind to human cells. They said this ability could have been acquired from a virus found in pangolins – a scaly mammal that is one of the most illegally trafficked animals in the world.

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Dr Elena Giorgi, of Los Alamos national laboratory, one of the study’s lead authors, said people had already looked at the pangolin link but scientists were still divided about their role in the evolution of Sars-Cov-2, the virus that causes Covid-19.

“In our study, we demonstrated that indeed Sars-Cov-2 has a rich evolutionary history that included a reshuffling of genetic material between bat and pangolin coronavirus before it acquired its ability to jump to humans,” she said, adding that “close proximity of animals of different species in a wet market setting may increase the potential for cross-species spillover infections”.

However, Prof Edward Holmes, an evolutionary biologist at the University of Sydney, in Australia, said more work on the subject was needed. “There is a clear evolutionary gap between Sars-Cov-2 and its closest relatives found to date in bats and pangolins,” he said. “The only way this gap will be filled is through more wildlife sampling.”

The findings came as Donald Trump announced that the United States was severing its ties with the World Health Organization because it had “failed to reform”.

In a speech at the White House devoted mainly to attacking China for its alleged shortcomings in tackling the initial outbreak of coronavirus, Trump said: “We will be today terminating our relationship with the World Health Organization and redirecting those funds to other worldwide and deserving urgent global public health needs.”

The US is the biggest funder of the WHO, paying about $450m (£365m) in membership dues and voluntary contributions for specific programmes.

Trump’s declaration was condemned in the US and around the world, with Australian experts joining counterparts in the UK and elsewhere in voicing their support for the WHO. Prof Peter Doherty, a Nobel laureate and patron of the Doherty Institute, which is part of global efforts to find a Covid-19 vaccine, said the WHO had the “full support of the scientific community”.

Deaths in the US have climbed to more than 102,000, with 1,747,000 infections. It is by far the biggest total in the world. On Friday it emerged that one person who attended the controversial pool parties in the Ozarks last weekend had tested positive for the virus.

In Brazil, there was another large rise in deaths. More than 27,000 people have died from the disease and the country has the world’s second highest number of cases, at 465,000.

There were also big surges in reported deaths in Russia, which identified more coronavirus cases in a day than at any time since early April; 2,819 more people tested positive on Friday.

Iran also recorded its biggest daily increase in deaths – 232 in 24 hours – bringing the total to 4,374. President Hassan Rouhani nevertheless said mosques were to resume daily prayers throughout the country, despite some areas reporting continuing high levels of infections. He added that physical distancing and other health protocols would be observed in mosques. He did not say when they were due to reopen.

India, meanwhile, reported a record daily jump of 7,964 new infections. With the latest tally, India has now reported 173,763 coronavirus cases and 4,971 deaths, making it the ninth most-affected country, according to Reuters. While the fatality rates in India have been lower than in worse-hit countries, experts fear the peak has not been reached. The latest numbers would appear to confirm that prediction.

Egypt registered 1,289 new cases and 34 deaths, the health ministry said, marking another record of daily increases on both counts despite stricter curfew rules.

Other developments across the world include:

• A leading UK government adviser has warned that it is too early to lift lockdown restrictions as planned next month because the number of new infections is still too high. John Edmunds, a professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, said he wanted the level of new cases to be “driven down further” before larger gatherings are allowed as the government has said it wants to do. Tory MPs are still being bombarded by constituents with calls for Boris Johnson’s top adviser to quit after he appeared to breach lockdown rules.
• Restrictions continue to be lifted to some degree across Europe, with thousands flocking to open-air cinemas to see films together for the first time in weeks.
• In Australia, where states are expected to move to relax the rules to allow gatherings of more people from Monday, anti-vaccine protesters gathered in several cities to claim that they believed Covid-19 was a “scam”.
• Also in Australia, scientists are examining the sewage waste in a town in Queensland where a 30-year-old man died this week from the virus. Nathan Turner is the youngest victim in the country so far and the case has baffled experts because he had not left the remote town of Blackwater.
• The global death toll passed 365,000, according to data compiled by Johns Hopkins University, with the number of cases just short of 6 million. The true number of infections is likely to be much higher, however, given the vast number of unrecorded and asymptomatic cases.