The new US administration has already said it intends to cut funding for the Green Climate Fund and related fora.
The Paris Agreement struck in 2015 to limit warming by capping emissions from burning coal, oil and gas, is "our lifeline", pleaded the Climate Vulnerable Forum.
The grouping represents the interests at UN climate negotiations of over a billion people in nearly 50 countries on five continents.
"As long there is a chance to stop global warming at a level that lets humanity survive and thrive, we should seize it," CVF representative Emmanuel De Guzman, a climate commissioner from the Philippines, said on the sidelines of UN talks under way in Bonn.
"This is why we continue to advance the call for world leaders to keep to the 1.5 goal and to recalibrate climate finance" for poorer countries to build less polluting infrastructure and raise their defences against climate impacts.
The Paris Agreement set a limit of two degrees Celsius (3.6 degrees Fahrenheit) limit for average global warming over pre-Industrial Revolution levels.
Also underwritten is an aspirational lower target of 1.5 C, which the CVF considers says is key to the survival of millions of its people.
Trump has yet to announce whether or not he intends to execute his threats to withdraw America from the pact which his predecessor, Barack Obama, was instrumental in pushing through.
"We really believe that right now without increased climate action no country can ever be great again," said De Guzman, referring to Trump's campaign slogan: "Make America great again."
The American president may also opt to abandon the US' pledge to reduce emissions by 26-28 percent below 2005 levels by 2025.
The new administration has already said it intends to cut funding for the Green Climate Fund and related fora, including the UN climate secretariat under whose auspices the 196-nation Paris Agreement was negotiated.
"There should be no backsliding on existing commitments," said a CVF statement, which warned that "inaction is a serious threat to global cooperation."
Trump is only expected to make his announcement after returning from a meeting of the G7 rich nations in Sicily on May 26 and 27, where many are hoping America's peers will put pressure on Trump to stay in the deal.
The other six, Britain, Canada, France, Germany, Italy and Japan, "must make a strong case for action," argued climate activist Mohamed Adow of Christian Aid, which advocates for poor country causes at the UN forum.
"Like witnesses to a violent assault, they will be complicit in the suffering of the world’s poor if they refuse to try and steer America back towards the right path."
The Paris Agreement's signatories, including a delegation from the United States, are gathered in Bonn until Thursday to work on a nuts-and-bolts "rulebook" for achieving the agreement's goals.