NEW YORK (NYTIMES) - A small group of scientists and others who believe the coronavirus that spawned the pandemic could have originated from a lab leak or accident is calling for an inquiry independent of the World Health Organisation's team of independent experts sent to China last month.

While many scientists involved in researching the origins of the virus continue to assert that the Sars-CoV-2 pandemic almost certainly began in a leap from bats to an intermediate animal to humans, other theories persist and have gained new visibility with the WHO-led team of experts' visit to China.

Officials with the WHO have said in recent interviews that it was "extremely unlikely" but not impossible that the spread of the virus was linked to some lab accident.

The open letter, first reported in The Wall Street Journal and the French publication Le Monde, lists what the signers see as flaws in the joint WHO-China inquiry and state that it could not adequately address the possibility that the virus leaked from a lab.

The letter further posits the type of investigation that would be adequate, including full access to records within China.

The WHO mission, as with everything involving China and the coronavirus, has been political from the start, as the international team's members acknowledged.

Dr Richard Ebright, a molecular biologist at Rutgers University and one of the scientists who signed the letter, said it grew out of a series of online discussions among scientists, policy experts and others who came to be known informally as the Paris group.

Many of those who signed the letter were based in France, and Dr Ebright, who has been outspoken about the need to investigate a possible laboratory leak, said such discussion had been less vigorous in the United States.

He said that no one in the group thought that the virus had been intentionally created as a weapon, but they were all convinced that an origin in a lab through research or by accidental infection was as likely as a spillover occurring in nature from animals to humans.

Dr Ebright said the letter was released because the Paris group expected to see an interim report from the WHO on Thursday (March 4). The letter, he added, "was communicated to high levels of the WHO on Tuesday".

Asked to respond to the letter, Mr Tarik Jasarevic, a spokesman for the WHO, replied in an e-mail that the team of experts that had gone to China "is working on its full report as well as an accompanying summary report, which we understand will be issued simultaneously in a couple of weeks."

The open letter noted that the WHO's study was a joint effort by a team of outside experts, selected by the global health organisation, who worked along with Chinese scientists, and that the team's report must be agreed on by all. The letter emphasised that the team was denied access to some records and did not investigate laboratories in China.

Findings by the team, the letter stated, "while potentially useful to a limited extent, represent neither the official position of the WHO nor the result of an unrestricted, independent investigation".

Dr Filippa Lentzos, a senior lecturer in science and international security at King's College London and one of the signers of the letter, said, "I think in order to get a credible investigation, it has to be more of a global effort, in the sense that it should be taken to the UN General Assembly, where all the nations of the world are represented and can vote on whether or not to give a mandate to the UN secretary-general to carry out this kind of investigation."

Dr David Relman, a professor of medicine and microbiology at Stanford University and a member of the intelligence community studies board at the National Academies of Science, Engineering and Medicine, an advisory body to the federal government, said he was "quite supportive" of the open letter.

"I completely agree, based on what we know so far, that the WHO investigation appears to be biased, skewed, and insufficient," he said in an e-mail. "Most importantly, without full transparency and access to the primary data and records, we cannot understand the basis for any of the comments issued so far on behalf of the investigation or by WHO."

At the same time, scientists working on coronaviruses continue to unearth and report evidence to support the natural evolution and spillover of the virus from animals.

Dr Robert Garry, a virus expert at Tulane University Medical Centre, recently posted on the website Virological a report that is not yet peer-reviewed that described new evidence that aspects of the virus that seemed unusual at first had been found in new viruses in Japan, Thailand and Cambodia.

He and his co-authors concluded: "These observations are consistent with the natural origin of Sars-CoV-2 and strongly inconsistent with a laboratory origin."

He said that he was familiar with some of the views of the letter signers expressed in previous media appearances or on social media, involving speculation about ways the virus could have come from laboratory work, and that none of those views appeared in the letter.