UN Secretary-General Antonio Guterres said it's "absolutely essential" for the world to unite to fight the threats arising from climate change.
His statement comes after US President Donald Trump's recent refusal to join leaders of the G7 countries to reaffirm their support for the 2015 Paris climate change agreement.
PM Narendra Modi has meanwhile offered India's support for climate change mitigation efforts.
Trump doesn't support climate change agreement, will clarify US stance
Trump had once called climate change a "hoax" concocted by the Chinese. He previously threatened to pull out of the Paris accord.
The US has ratified the accord but Trump contends that the agreement to cut emissions is bad for the American economy.
Last week, Trump refused to reaffirm the US' commitment to the accord and said he would clarify America's stance soon.
Guterres says rest of world must continue supporting climate deal
Speaking in New York, Guterres said: "The world is in a mess. It is absolutely essential that the world implements the Paris agreement."
Guterres said if any country doubted the agreement's need, then other countries "must unite even stronger and stay the course."
"The message is simple: the sustainability train has left the station. Get on board or get left behind," he added.
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Guterres: 'Important for America not to leave Paris agreement'
Guterres said: "We believe that it would be important for the US not to leave the Paris agreement.
"But even if the government decides to leave the Paris agreement, it's very important for US society as a whole… to remain engaged," he added.
The US states of New York and California have already pledged to fight climate change, regardless of the Trump administration's support.
Modi says spoiling environment for future generations 'a crime'
Guterres's stance was echoed by Modi, who spoke at a joint press conference with German Chancellor Angela Merkel in Berlin.
Both Modi and Merkel also reaffirmed their support for joint Indo-German cooperation in areas of developing climate-friendly energy solutions.
Reports: Trump to withdraw US from Paris Agreement on climate-change
However, Trump tweeted he will announce his decision on the Paris Accord soon.
The exit of US, one of the largest carbon dioxide emitters, would be a huge blow to the deal that aims to avert the adverse affects of climate change.
Why Modi's statement on climate change matters
Trump's stance on climate change has left Europe and the rest of the world dismayed.
Modi's statement can be seen as New Delhi's continued support for the climate change agreement.
This is crucial for the accord's success as India is one of the world's largest polluters.
India's support is also significant for global diplomatic efforts to isolate the US on the climate change issue.
Paris Climate Agreement
United Nations (UN)