A leading scientist on the World Health Organization's (WHO) COVID-19 mission to China said on Tuesday that a follow-up trip could be helpful to gather additional research on the origins of the disease, but should be separate from any audit of information provided by Beijing.

The comments from Dutch virologist Marion Koopmans came after the United States called on Tuesday for international experts to be allowed to evaluate the source of the coronavirus and the early days of the outbreak in a second phase of the WHO's investigation into the origins of the coronavirus.

Koopmans was part of the WHO-led team which spent four weeks in China earlier this year and in March published a report jointly with Chinese scientists that said the virus had probably been transmitted from bats to humans through another animal.

"Introduction through a laboratory incident was considered to be an extremely unlikely pathway," its report said.

Discussions about the outbreak gained renewed attention this week as U.S. intelligence agencies examine reports that researchers at a Chinese virology laboratory in Wuhan were seriously ill in 2019 a month before the first cases of COVID-19 were reported.

U.S. government sources have said there is still no proof the disease originated at the lab.

Koopmans said the team would be eager to carry out additional research in China in a number of areas and was awaiting the outcome of WHO discussions. She stressed the need for a clear mandate to conduct research, not to carry out an audit.

"I think these cannot be combined. So we believe that is a combination that will not work. In that case you say we are going to carry out an inspection, or we are going to do the follow-up research, or both, but through different mechanisms, otherwise you simply will not make any progress," she said.