The US envoy to China has called on the country to be "more honest" about the origins of the Covid-19 virus.

His comments come a day after US media reported a federal agency had found the pandemic probably started from a laboratory leak in Wuhan.

China's foreign ministry hit back that the global outbreak's origin "was about science and should not be politicised".

Washington-Beijing ties have been under strain since the US shot down an alleged Chinese spy balloon this month.

Ambassador Nicholas Burns told a US Chamber of Commerce event on Monday that China needs to "be more honest about what happened three years ago in Wuhan with the origin of the Covid-19 crisis".

US media reported on Sunday that a report from the US Department of Energy had concluded in a classified intelligence report with "low confidence" that the virus was accidentally leaked by a laboratory.

The energy department had formerly said it was undecided on how the virus began.

Other US agencies have drawn differing conclusions, with varying degrees of confidence in their findings. The FBI in 2021 concluded with "moderate confidence" that the virus leaked from a lab.

Other studies suggest it made the leap from animals to humans at Wuhan's Huanan seafood and wildlife market.

An unclassified report released by the US top spy official in October 2021 said that four US intelligence agencies had assessed with "low confidence" that it had originated with an infected animal or a related virus.

White House national security spokesman John Kirby said on Monday there was still no firm finding either way.

"There has not been a definitive conclusion and consensus in the US government on the origins of the Covid-19 pandemic," he told reporters when asked about the energy department's reported determination.

Covid: Woman caught virus twice within record 20 days

A 31-year-old healthcare worker caught Covid twice within 20 days - the shortest-known gap between infections, Spanish researchers have claimed.

Tests show the woman was infected with two different variants - Delta in late December and then Omicron in January.

This shows that even if you have had Covid before, you can still be infected again even if fully vaccinated, the researchers say.

Based on that definition, health officials say nearly 900,000 people in the UK have potentially been infected twice with Covid up to the start of April.

It is difficult to pin down an exact number, because only whole genome sequencing can confirm the infections are caused by different strains, and very few positive tests go through this process.

The Spaniard did not develop any symptoms after her first positive PCR test, but less than three weeks later she developed a cough and fever which prompted her to take another test.

When the tests were analysed further, they showed the patient had been infected by two different strains of coronavirus.

In a presentation at the European Congress of Clinical Microbiology and Infectious Diseases, study author Dr. Gemma Recio said the case highlighted that Omicron can "evade the previous immunity acquired either from a natural infection with other variants or from vaccines".

She said: "In other words, people who have had Covid-19 cannot assume they are protected against reinfection, even if they have been fully vaccinated.

"Nevertheless, both previous infection with other variants and vaccination do seem to partially protect against severe disease and hospitalisation in those with Omicron," added Dr Recio, from the Institut Catala de Salut, Tarragona in Spain.