US President Joe Biden signed executive orders to launch his administration on Wednesday, including a decision to rejoin the Paris climate accord and a mask mandate for all federal buildings.
The orders included keeping the US in the World Health Organisation, ending the ban on entries from mostly Muslim-majority countries, bolstering environmental protection and strengthening the fight against Covid-19.
Also among the directives was a halt on building the wall on the Mexico border and moves to expand diversity and equality for minority groups in the federal government.
The orders were aimed at reversing decisions by Donald Trump and setting a clear policy path for Mr Biden's administration, just hours after he was sworn in as president.
"Some of the things we are going to be doing are going to be bold," he said in the Oval Office.
"We are going to combat climate change in a way we have not done so far."
Mr Biden said that included returning to the Paris agreement, a treaty signed by most nations and the EU in 2015 and put into effect in 2016 to limit global warming.
He said his action on the Covid-19 pandemic, which has claimed 400,000 lives in the US, would help to change its course.
His return to the Paris agreement, which the US joined when Barack Obama was president and Mr Biden was vice president, was praised by other leaders.
"Welcome back," said French President Emmanuel Macron. "We are together. We will be stronger to face the challenges of our time. Stronger to build our future. Stronger to protect our planet."
UN Secretary General Antonio Guterres also welcomed the move, saying it was a big step for the leading producer of global carbon pollution.
"But there is a very long way to go," Mr Guterres said. "We look forward to the leadership of the United States in accelerating global efforts towards net zero" emissions.
He called for ambitious new targets for 2030 and more financing to battle climate change.
Mr Biden's top climate official, John Kerry, praised the move as a boon for "America's credibility and commitment – setting a floor, not a ceiling, for our climate leadership".