China has rejected the next stage of a World Health Organization (WHO) plan to investigate the origins of the coronavirus pandemic.
The WHO wants to audit laboratories in the area the virus was first identified.
WHO experts said it was very unlikely the virus escaped from a Chinese lab, but the theory has endured.
Investigators were able to visit Wuhan - the city where the virus was first detected in December 2019 - in January this year.
But earlier this month WHO head Dr Tedros Adhanom Ghebreyesus outlined the terms of the inquiry's next phase. This included looking at certain science research institutions.
He has now called on China to be more co-operative about the early stages of the outbreak.
He urged Beijing to "be transparent, to be open and co-operate" with investigators and provide raw patient data that had not been shared during the first probe.
He said it was "impossible" for China to accept the terms, adding that the country had submitted its own origins-tracing recommendations.
"We hope the WHO would seriously review the considerations and suggestions made by Chinese experts and truly treat the origin tracing of the Covid-19 virus as a scientific matter, and get rid of political interference," Reuters quoted Mr Zeng as saying.
Yuan Zhiming, director of the National Biosafety Laboratory at the Wuhan Institute of Virology, also appeared at the press conference. He said the virus was of natural origin and maintained no virus leak or staff infections had occurred at the facility since it opened in 2018.
More than four million people have died worldwide since the start of the pandemic and the WHO has faced growing international pressure to further investigate the origins of the virus.
Earlier this year, US President Joe Biden also ordered American intelligence officials to "redouble" their own investigative efforts into the pandemic.