President Joe Biden's top medical adviser on Covid-19 has announced renewed US support for the World Health Organisation (WHO) after it faced blistering criticism from the Trump administration.

Dr Anthony Fauci, speaking by video conference from a pre-dawn United States to the WHO's executive board, said the US will join the UN health agency's efforts to bring vaccines, therapeutics and diagnostics to people in need, whether in rich or poor countries.

He said the US will also resume full funding and staffing support for the WHO.

Dr Fauci's quick commitment to the WHO - whose response to the coronavirus outbreak was repeatedly berated by the Trump administration - marks a dramatic and vocal shift towards a multilateral approach to fighting the pandemic.

"I am honoured to announce that the United States will remain a member of the World Health Organisation," Dr Fauci said.

Just hours after Mr Biden's inauguration on Wednesday, his administration announced the US will revoke a planned pullout from the WHO in July that had been announced by the Trump administration.

Dr Fauci said the Biden administration "will cease the drawdown of US staff seconded to the WHO" and resume "regular engagement" with the WHO.

"The United States also intends to fulfil its financial obligations to the organisation," he added.

He referred to Tedros Adhanom Ghebreyesus, the WHO director-general, as "my dear friend".

Other countries and the WHO chief jumped in to welcome the US announcements, and pledged to work with the Biden administration.

"This is a good day for WHO and a good day for global health," Mr Tedros said, referring to "my brother Tony" in reference to Dr Fauci, while congratulating Mr Biden and vice president Kamala Harris.

"The role of the United States, its role, global role is very, very crucial."

The United States under Mr Trump had been the highest profile - and most deep-pocketed - holdout from the Covax Facility, which has faced financial hurdles and contractual challenges with vaccine manufacturers as well as vast logistical issues.

Both the European Union and China have given their support to the programme.