Climate negotiators return for talks in Bonn on Tuesday under the cloud of Donald Trump's threats to pull America from the hard-fought Paris Agreement, as the president prepares to hold a special meeting to discuss US participation.
After months of uncertainty, the US president appeared closer to a decision on whether he intends to keep a campaign promise to withdraw Washington from the climate-rescue pact, whose practicalities are being haggled over during UN talks in Bonn.
Trump and his top climate and economic aids will kick-off discussions in earnest on Tuesday in the US, a senior administration official said, adding that "they are meeting tomorrow at 1.30 pm".
Uncertainty over America's future has already loomed large during the 11-day meeting to work out the nuts and bolts of implementing the international deal, which Trump's predecessor, Barack Obama, was instrumental in bringing about.
David Balton, the US deputy assistant secretary for international environmental affairs, said Monday: "The last thing I heard is that the president, our president, has indicated that he plans to make a decision some time over the next couple of weeks, but not this week."
A total of 196 countries are now parties to the climate deal, clinched in 2015 after years of tough bartering, which Trump threatened to "cancel".
The May 8-18 Bonn meeting is meant to start drafting a guide for member countries to execute the pact, which seeks to brake global warming by curbing fossil fuel emissions.
But the negotiations risk being hamstrung over fears that the world's number two carbon polluter will pull out and throw the pact into disarray.
"There's no question that if the US withdraws it is going to create difficulties... in the negotiations," Paula Caballero of the World Resources Institute think-tank said as climate envoys met for their first session since Trump took office.
The US did send a delegation to the talks, led by Obama-era negotiator Trigg Talley, who declined to comment on their brief.
A State Department official told AFP: "We are focused on ensuring that decisions are not taken at these meetings that would prejudice our future policy, undermine the competitiveness of US businesses, or hamper our broader objective of advancing US economic growth and prosperity."
Numerous speakers at a plenary session Monday stressed that the deal cannot be "renegotiated" -- a proposal of Trump's Energy Secretary Rick Perry.
Widely hailed as the last chance to stave off worst-case-scenario global warming, the Paris Agreement was savaged by a campaigning Trump, who called climate change a "hoax" perpetrated by China.
With the rest of the world waiting ever since, Trump has said he will make his decision before the next G7 meeting on May 26-27 in Sicily.
Salaheddine Mezouar of Morocco, who presided over the last high-level climate negotiations in 2016, said he trusted that "the good sense and pragmatic spirit President Trump and his team" would prevail.
"It would be difficult, crazy, to go against public opinion, against the domestic will, and the will of the international community," Mezouar said in Bonn.
Some fear a US U-turn would dampen enthusiasm among other signatories for more efforts on emissions-cutting targets.
This is key, as current pledges place the world on track for warming much higher than the ceiling of two degrees Celsius (3.6 degrees Fahrenheit) written into the agreement.