BONN - UN climate negotiators in Bonn 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 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 keeping a campaign promise to withdraw Washington from the Paris Agreement.

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 a campaigning Trump, who threatened to "cancel" it if elected.

With the rest of the world waiting for finality 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, have 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.

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."

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.

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 rich nations have promised in climate aid.

The new 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 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."

US, China must 'lead the

way' on climate: Obama

The United States and China have to lead the way in the fight against climate change, former US president Barack Obama said Tuesday.

Speaking as the future of the landmark Paris climate accord hangs in the balance, he acknowledged differences with his successor Donald Trump but said the private sector also had a role to play.

"It's important that big countries that are big emitters like the US and China... lead the way," he told a conference in the Italian city of Milan, saying developing countries were watching them.

"The current administration has differences with my administration in terms of environmental policy," he said, referring to Trump.

"The good news is the private sector has already made a determination that the future is clean energy. Those things are locked in now, into the energy sector."

"Because of the debates taking place in the current administration the steps may be taken more slowly than they would have been done, but I'm confident," he added.

While campaigning last year, Trump called climate change a hoax perpetrated by China, and promised to cancel the deal if he won the presidency.

Now, after months of uncertainty, he appears to be edging toward a decision on whether to honour the landmark 2015 agreement to limit global warming.

A key White House meeting scheduled for Tuesday to discuss whether the United States will honour the Paris accord was postponed, with no new date set.