A senior administration official confirmed that the White House meeting to discuss the issue has been postponed.
UN climate negotiators in Bonn, Germany were left frustrated Tuesday as the White House postponed a meeting to determine whether the US will stay in the 196-nation Paris Agreement to curb planet-harming fossil fuel.
As uncertainty mounted over the hard-fought pact's future under US President Donald Trump, China's leader Xi Jinping came to its defence.
China and France "should protect the achievements of global governance, including the Paris Agreement," the foreign ministry in Beijing quoted Xi as telling his newly-elected counterpart Emmanuel Macron in a phone call.
Trump's predecessor Barack Obama, who alongside Xi was instrumental in the agreement's birth in 2015, also entered the fray on Tuesday.
Big pollution emitters like the US and China, he said, must "lead the way" in the fight against climate change.
China is the world's biggest emitter of greenhouse gases from burning coal, oil and gas with about 25 percent of the global total, followed by the US with around 15 percent.
Trump, who has described climate change as a "hoax" perpetrated by China, has yet to announce whether he intends on keeping a campaign promise to withdraw Washington from the Paris Agreement.
It's been postponed
Negotiators in Bonn had their eyes firmly on a White House meeting called to discuss the topic on Tuesday, but a senior administration official confirmed, "It's been postponed."
No new date was given.
The May 8-18 Bonn meeting is meant to start designing a "rulebook" for implementing the global deal to limit average global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.
This is the ceiling at which scientists say the planet can avoid worst-case-scenario climate change impacts, rising seas, harsher droughts, more intense storms, disease-spread and conflict over dwindling natural resources.
The agreement was savaged by Trump during last year's US presidential election campaign, during which he threatened to "cancel" it if elected.
With the rest of the world awaiting a definitive US position ever since, the new president has said he will make his decision before the next G7 meeting on May 26-27 in Sicily.
The Bonn meeting, a technical discussion held every year to prepare for an annual round of political-level negotiations, has been overshadowed by fears that a US withdrawal would throw the entire process into disarray.
At an open session late Monday, speaker after speaker reiterated the deal must not be "renegotiated," a proposal of Trump's Energy Secretary Rick Perry.
"It is definitely the elephant in the room," a member of one national delegation told AFP on Tuesday. "But it is not on the agenda, so we cannot discuss it."
The US did send a delegation to the talks, led by Obama-era negotiator Trigg Talley who declined to comment on its brief.
"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," a State Department official told AFP.
Low-key US team
Some fear a US U-turn would dampen enthusiasm among other signatories to increase their 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 written into the agreement.
"If Trump withdraws from the accord, it will be a hard blow but not a surprise," Seyni Nafo, chief negotiator for the Africa Group, told AFP.
"It risks undermining the dynamic" of give-and-take built up over years in the UN climate negotiations process.
An American withdrawal would threaten not only its own pledge of cutting emissions 26-28 percent from 2005 levels by 2025, but also those of poorer countries which depend on funds that rich nations have promised in climate aid.
The new US administration has already moved to cut funding for climate bodies including the UN Framework Convention on Climate Change (UNFCCC) secretariat hosting the annual negotiations.
American participation in Bonn was described as low-key by colleagues.
And there has already been a spillover effect.
"There are countries which are more cautious," in making proposals, said a delegate. "They are taking a wait-and-see approach."