US President Joe Biden has begun to undo some of Donald Trump's key policies, hours after being sworn in.
In his initial acts as the 46th US president, he signed 15 executive orders - the first to boost the federal response to the coronavirus crisis.
Other orders reversed the Trump administration's stance on climate change and immigration.
President Biden set to work at the Oval Office having been sworn in earlier on Wednesday at the US Capitol.
President Biden "will take action - not just to reverse the gravest damages of the Trump administration - but also to start moving our country forward," a statement detailing his executive orders said.
On coronavirus, a series of measures will be enacted to tackle the pandemic which has claimed more than 400,000 lives in the US.
There will be a mandate to wear masks and practise social distancing on all federal government property.
A new office will be set up to co-ordinate the response to the pandemic and the US will halt the process - begun by the Trump administration - of withdrawing from the World Health Organization (WHO).
The move to re-engage with the WHO was welcomed by UN Secretary-General Antonio Guterres who said it was "absolutely critical" for a more co-ordinated global response, his spokesman Stéphane Dujarric said.
Mr Biden has also pledged to make the fight against climate change a top priority of his administration.
He signed an executive order beginning the process of rejoining the 2015 Paris climate agreement, from which Mr Trump formally withdrew the US last year.
Mr Biden's climate envoy, former US Secretary of State John Kerry, tweeted that the commitment set "a floor, not a ceiling" for America's climate leadership and urged international co-operation ahead of the UN Climate Change Conference (Cop26) in Glasgow in November.
Additionally, Biden will issue a directive on Thursday including the intent to join the COVAX vaccine facility that aims to deliver vaccines to poor countries, his chief medical adviser, Anthony Fauci, told the WHO.
Other issues to be addressed by the new administration over the next 10 days include healthcare, the economy, immigration and climate change.