Discovering the origins of COVID-19 is a moral imperative and all hypotheses must be explored, the head of the World Health Organization said, in his strongest comments yet that the U.N. body remains committed to finding how the virus arose.

A U.S. agency was reported by the Wall Street Journal to have assessed the pandemic had likely been caused by an unintended Chinese laboratory leak, raising pressure on the WHO to come up with answers. Beijing denies the assessment.

"Understanding #COVID19's origins and exploring all hypotheses remains: a scientific imperative, to help us prevent future outbreaks (and) a moral imperative, for the sake of the millions of people who died and those who live with #LongCOVID," Tedros Adhanom Ghebreyesus said on Twitter late on Saturday.

He was writing to mark three years since the WHO first used the word "pandemic" to describe the global outbreak of COVID-19.

Activists, politicians and academics said in an open letter this weekend that the focus of the anniversary should be on preventing a repeat of the unequal COVID-19 vaccine rollout, saying this led to at least 1.3 million preventable deaths.

In 2021, a WHO-led team spent weeks in and around Wuhan, China where the first human cases were reported and said in a joint report that the virus had probably been transmitted from bats to humans through another animal but further research was needed. China has said no more visits are needed.

Since then, the WHO has set up a scientific advisory group on dangerous pathogens but it has not yet reached any conclusions on how the pandemic began, saying key pieces of data are missing.