LONDON: The origin of the novel coronavirus is still unclear and there is not yet enough evidence to say conclusively if it occurred naturally or was caused by a laboratory leak, a group of leading scientists said in a letter.

The novel coronavirus, which emerged in China in late 2019, has killed 3.34 million people, cost the world trillions of dollars in lost income and upended daily life for billions of people.

"More investigation is still needed to determine the origin of the pandemic," said the 18 scientists, including Ravindra Gupta, a clinical microbiologist at the University of Cambridge and Jesse Bloom, who studies the evolution of viruses at the Fred Hutchinson Cancer Research Center.

"Theories of accidental release from a lab and zoonotic spillover both remain viable," the scientists including David Relman, professor of microbiology at Stanford, said in a letter to the journal Science.

The authors of the letter said the World Health Organization's initial investigation into the origins of the virus had not made a "balanced consideration" of the theory that it may have come from a laboratory incident.

"We must take hypotheses about both natural and laboratory spillovers seriously until we have sufficient data," the scientists said, adding that an intellectually rigorous and dispassionate investigation needed to take place.

"In this time of unfortunate anti-Asian sentiment in some countries, we note that at the beginning of the pandemic, it was Chinese doctors, scientists, journalists, and citizens who shared with the world crucial information about the spread of the virus—often at great personal cost."