Countries have the responsibility to work together, in partnership with the World Health Organisation (WHO), so scientists can try to find the origin of the Covid-19 pandemic, the UN health agency said Friday after China rejected the plan for a second phase of investigations the day before.

WHO's Director-General Tedros Ghebreyesus had agreed last week to a call for further studies after the release of an inconclusive report on an international team's field visit to China's Wuhan city to research the origins of Covid-19, citing difficulties accessing raw data. The first human cases were identified in Wuhan.

"Countries have the responsibility to work together and to work with the WHO in the spirit of partnership, so scientists can have their space to try to find the origins of this pandemic," WHO spokesperson Tarik Jasarevic told the media.

Read: China rebuffs WHO’s terms for further COVID-19 origins study

"Now countries are looking into the proposal that the director-general outlined last week. They need some time. We look forward to input and constructive dialogue," he added.

However, Zeng Yixin, deputy head of China's National Health Commission, had said, "The work plan on the second phase origins study proposed by the WHO contains language that does not respect science."

On July 16, Tedros said he expects China to support the next phase of the scientific process to identify the origins of SARS-CoV-2.

"Finding where this virus came from is essential not just for understanding how the pandemic started and preventing future outbreaks, but it is also important as an obligation to the families of the 4 million people who have lost someone they love, and the millions who have suffered," said Tedros.

Read: Natural origins theory of Covid-19 still the most likely: Fauci

"We expect China to support this next phase of the scientific process by sharing all relevant data in a spirit of transparency. Also, the WHO expects all member states to support the scientific process by refraining from politicising it," he added.

At the end of March, a WHO-led international scientific team delivered its report following a mission to China in January, in line with the World Health Assembly for a probe into the virus.