BONN - World leaders descended on Bonn Wednesday to re-energise climate talks hamstrung by America's rejection of a planet rescue plan the rest of the world is fighting hard to put into action.

Labelling climate change "the defining threat of our time", UN chief Antonio Guterres said continued investment in atmosphere-fouling fossil fuels guaranteed an "unsustainable future".

"Ultimately, there is only one ambition that matters - to build a secure world of peace, prosperity, dignity and opportunity for all people on a healthy planet," he told dignitaries and bureaucrats at the annual UN climate conference. "The world counts on your wisdom and foresight."

Despite announcing in June it will withdraw from the Paris Agreement, the United States has a delegation at the Bonn talks, where rules for executing the pact on winding down Earth-warming greenhouse gas emissions from burning coal, oil and gas are being drawn up.

The US presence is not universally appreciated, especially after White House officials hosted a sideline event on Monday defending continued fossil fuel use.

On Wednesday, Guterres, French President Emmanuel Macron and German Chancellor Angela Merkel arrived to lead a diplomatic drive to revitalise negotiations that have become stuck on finance - an old bugbear of the UN process.

Macron and Merkel were due to address the conference in the afternoon in the presence of some 25 heads of state and government and dozens of ministers of energy and the environment.

The United States was represented at the highest level by an acting assistant secretary of state, Judith Garber, who will address delegates on Thursday.

The United States, which championed the Paris Agreement under former president Barack Obama, ratified it just two months before Donald Trump - who has described climate change as a "hoax" - was voted into office.

This week, Syria became the 196th country to formally adopt the hard-fought agreement, leaving America as the only nation in the UN climate convention to reject it.

The Paris Agreement commits countries to limiting average global warming to under two degrees Celsius (3.6 degrees Fahrenheit) over Industrial Revolution levels, and 1.5 C if possible, to avert calamitous climate change-induced storms, drought and sea-level rises.

To bolster the agreement, nations submitted voluntary commitments to curb greenhouse gas emissions from fossil fuel burning.

But the 1 C mark has already been passed, and scientists say that on current country pledges, the world is headed for a 3 C warmer future, or more.

For the past nine days, bureaucrats have been haggling over a Paris Agreement "rulebook", which will specify how countries must calculate and report their contribution to global emissions cuts.

From Wednesday, it is the turn of energy and environment ministers to unlock issues above the pay grade of rank-and-file negotiators.

A report released Wednesday warned that America's withdrawal from the pact will push global temperatures nearly half a degree Celsius higher by 2100, for a total of 3.2 C.

To bring home the urgency, conference organisers put a 12-year-old boy from Fiji's climate change-affected province of Tailevo on stage.

Timoci Naulusala told delegates that last year his village was hit by one of the strongest cyclones in history.

"My home, my school, my source of food, water, money was totally destroyed. My life was in chaos," he said. "My once beautiful village is now a barren and empty wasteland."

Merkel will be closely watched, with observers hoping she will announce a phase-out of coal in her country.

The commodity provides about 40 percent of Germany's electricity needs, and the country is set to miss its own goal of cutting emissions by 40 percent by 2020 from 1990 levels.

The issue has dogged Merkel's efforts to form a coalition government between her conservative allies, the anti-coal Greens, and pro-industry Free Democrats.

"Chancellor Merkel over the years has been a great climate champion and has driven the global debate forward," said Jennifer Morgan, executive director of Greenpeace. "But that credibility is hanging in the balance."

A smattering of activists welcomed delegates to the conference Wednesday, with placards reading "End Coal", "Clean coal is a dirty lie", and "Stop corporate capture of the climate."

A key issue for ministers to resolve is a demand from developing countries for firm financing commitments to help them prepare for, and deal with, the fallout from climate change.

Rich countries, they say, carry more of the blame - having polluted more, for longer.

"It is crucial for developing countries to have predictability of finance," South Africa's environment minister Edna Molewa said Wednesday.