Samples from a study suggesting the coronavirus was circulating outside China by October 2019 have been re-tested at the World Health Organization's (WTO) request, two scientists who led the Italian research said.

There is growing international pressure to learn more about the origins of the pandemic that has killed more than 3 million people worldwide and U.S. President Joe Biden last week ordered his aides to find answers.

The WHO said on Friday experts were preparing a proposal on the next studies to be carried out into the origins of the virus, but that there was no set timeline. 

The UN body reacted to Biden's announcement that intelligence agencies were pursuing rival theories, including the possibility of a laboratory accident in China, by saying the search was being "poisoned by politics".