There has been widespread international condemnation of President Donald Trump's decision to withdraw the US from the 2015 Paris climate agreement.
German Chancellor Angela Merkel called the move "extremely regrettable" and said nothing would stop those who supported the accord.
But Russian President Vladimir Putin said he would "not judge" Mr Trump.
Mr Trump said he was prepared to discuss a new deal but key signatories to the accord quickly ruled that out.
He said the deal "punished" the US and would cost millions of American jobs. "I was elected to represent the citizens of Pittsburgh, not Paris," he quipped.
The Paris agreement commits the US and 194 other countries to keeping rising global temperatures "well below" 2C above pre-industrial levels and "endeavour to limit" them even more, to 1.5C.
The UN World Meteorological Organisation said on Friday that, in the worst scenario, the US pullout could add 0.3C to global temperatures by the end of the century.
Why is Trump pulling out?
Mr Trump characterised the Paris agreement as a deal that aimed to hobble, disadvantage and impoverish the US.
He said it would cost the US $3tn (£2.3tn) in lost GDP and 6.5 million jobs - while rival economies like China and India were treated more favourably.
Mr Trump said he was fulfilling his "solemn duty to protect America and its citizens".
He added: "We don't want other leaders and other countries laughing at us anymore - and they won't be."
What did they say about Trump?
"We need the Paris agreement to protect all of creation," said Chancellor Merkel. "Nothing can and nothing will stop us."
Mrs Merkel said the path set out by Paris was "irreversible", and she added: "We will travel it together."
"To everyone who cares about the future of our planet, I say let's continue on this path together to succeed in protecting Mother Earth," Mrs Merkel said.
In other criticism of Mr Trump's decision on Friday:
- European Commission President Jean-Claude Juncker, opening a summit with China in Brussels, said: "There is no backsliding on the Paris agreement"
- UN Secretary General Antonio Guterres said that the fight against climate change was "unstoppable"
- Indian Environment Minister Harsh Vardhan said: "As far as the Paris accord is concerned... our government is committed, irrespective of the stand of anyone, anywhere in the world"
- The group of 48 least developed countries were deeply disappointed but believed the global climate momentum would continue with or without the US, said chairperson Gebru Jember Endalew
- Small island nations whose existence is threatened by rising sea levels were also critical. The President of the Marshall Islands, Hilda Heine, said it was "highly concerning for those of us that live on the frontline of climate change"
- Japan's Finance Minister Taro Aso said: "I'm not just disappointed, but also feel anger"
Speaking in the Russian city of St Petersburg, President Putin said that while he would not judge Mr Trump, he thought the US should not abandon the Paris accord.
"This agreement hasn't even taken effect yet - it will take effect by 2021," he said, speaking in Russian. "So we still have time to reach a deal." Then, in English, he added: "Don't worry, be happy."
It is unclear which deadline Mr Putin was referring to as the Paris Agreement entered into force on 4 November 2016.
Can it all be renegotiated?
Mr Trump did not give a timescale on withdrawal. However, under the agreement, a nation seeking to leave the pact can only give notice three years after the agreement from the date it entered into force.
The process of leaving then takes another year, meaning it would not be complete until just weeks after the US presidential election in 2020.
US payments to the UN Green Climate Fund, which helps developing countries cope with the effects of climate change, will stop. The US has reportedly so far paid $1bn (£780m) of a $3bn pledge.
Mr Trump indicated he was open to another climate deal "on terms that are fair to the United States" but his words suggest it is not a priority for him.
"We will see if we can make a deal that's fair," he said. "And if we can, that's great. And if we can't, that's fine."
The leaders of France, Germany and Italy quickly issued a joint statement rejecting any renegotiation: "We deem the momentum generated in Paris in December 2015 irreversible and we firmly believe that the Paris agreement cannot be renegotiated, since it is a vital instrument for our planet, societies and economies."
What are they saying in America?
Barack Obama, who was president when the Paris deal was agreed, accused the Trump administration of "rejecting the future".
The Democratic governors of New York, California and Washington states all quickly vowed to respect the terms of the Paris deal.
Disney's chief executive Robert Iger and the entrepreneur Elon Musk both resigned from White House advisory councils in opposition to the decision.
"Climate change is real. Leaving Paris is not good for America or the world," said Mr Musk, the head of tech giant Tesla.
However, Republican congressional leaders and the US coal industry backed the move, with Senate Majority Leader Mitch McConnell supporting Mr Trump "for dealing yet another significant blow to the Obama administration's assault on domestic energy production and jobs".
Peabody Energy, America's biggest coal mining firm, said the agreement would have badly affected the US economy.
What was agreed in Paris?
Climate change, or global warming, refers to the damaging effect of gases, or emissions, released from industry, transportation, agriculture and other areas into the atmosphere.
The Paris accord is meant to limit the global rise in temperature attributed to emissions. Only Syria and Nicaragua did not sign up.
Countries agreed to:
- Keep global temperatures "well below" the level of 2C (3.6F) above pre-industrial times and "endeavour to limit" them even more, to 1.5C
- Limit the amount of greenhouse gases emitted by human activity to the same levels that trees, soil and oceans can absorb naturally, beginning at some point between 2050 and 2100
- Review each country's contribution to cutting emissions every five years so they scale up to the challenge
- Enable rich countries to help poorer nations by providing "climate finance" to adapt to climate change and switch to renewable energy
Nasa's Goddard Institute for Space Studies says the world's average temperature has risen by about 0.8C since 1880, two-thirds of that since 1975.
US think tank Climate Interactive predicts that if all nations fully achieve their Paris pledges, the average global surface temperature rise by 2100 will be 3.3C, or 3.6C without the US.