GENEVA: The World Health Organization (WHO) called on Friday for all countries to work together to investigate the origins of the coronavirus that caused COVID-19, a day after China rejected plans for more checks on labs and markets in its territory.

The first human cases of COVID-19 were reported in the central Chinese city of Wuhan in December 2019. China has repeatedly dismissed theories that the virus leaked from one of its laboratories.

The WHO this month proposed a follow-up to earlier investigations in China. But Zeng Yixin, vice minister of China's National Health Commission, said on Thursday Beijing would not accept the proposal as it stood.

When asked about China's rejection, WHO spokesman Tarik Jasarevic told a U.N. briefing in Geneva: "This is not about politics, it's not about a blame game.

"It is about basically a requirement we all have to try to understand how the pathogen came into the human population. In this sense, countries really have the responsibility to work together and to work with WHO in a spirit of partnership."

A WHO-led team spent four weeks in and around Wuhan with Chinese scientists and said in a joint report in March that the virus had probably been transmitted from bats to humans through another animal but that further research was needed.

WHO director-general Tedros Adhanom Ghebreyesus has said that the investigation was hampered by the lack of raw data on the first days of spread there.

Countries including the United States and some scientists have demanded further investigation, particularly into the Wuhan Institute of Virology, which was conducting research into bats.

Diplomats said that China immediately signalled opposition to the plan presented by Tedros at closed-door talks with member states a week ago.

"The Chinese see it as a repudiation of the joint report," said one.

Tedros also said last week he was establishing a permanent International Scientific Advisory Group for Origins of Novel Pathogens to help advance the next phase of studies into the origins of SARS-CoV-2.

The panel, to be composed of independent experts, is aimed at helping to ease some political pressure on the WHO, diplomats said.