A team of World Health Organization (WHO) investigating the origins of the deadly COVID-19 pandemic has left its quarantine hotel on Thursday to commence fieldwork.

The mission has been plagued by delay, concerns over access and bickering between China and the U.S., which has accused Beijing of hiding the extent of the initial outbreak and has criticized the terms of the visit, under which Chinese experts conducted the first phase of research. Meanwhile, the U.S demanded a "robust and clear" international probe into the origins of the coronavirus pandemic.

Beijing has so far frustrated international efforts to track the origins of the virus, which has killed more than 2.1 million people globally and only recently allowed the WHO team into China after repeated delays.

"It's imperative that we get to the bottom of the early days of the pandemic in China, and we've been supportive of an international investigation that we feel should be robust and clear," White House spokeswoman Jen Psaki said Wednesday.

President Joe Biden's predecessor Donald Trump had enraged China by accusing it of a botched response to the initial outbreak in Wuhan, and had led calls for an independent investigation.  China has sought to deflect blame for the massive global human and economic toll by suggesting,  without proof, that the virus emerged somewhere else.

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