MARRAKESH, Morocco - Donald Trump loomed large over climate talks on Tuesday where UN chief Ban Ki-moon urged nations to redouble their planet-rescue efforts and voiced hope the US would not pull out of its commitments.

Elected to the White House a week ago, Trump had previously called global warming a "hoax" perpetrated by China, and threatened to "cancel" the hard-fought Paris Agreement concluded a year ago.

But Ban said he had spoken to the businessman turned president-elect, and was "optimistic" Trump "will hear and understand the seriousness and urgency of addressing climate change".

"As the president of the US I am sure he will understand this, he will listen, he will vary his campaign remarks," the outgoing UN secretary general told journalists. "I am sure he will make a good, wise decision."

Gathered in Marrakesh since last Monday, representatives from the UN climate body's 197 parties have started thrashing out a roadmap for putting the agreement into action. But climate change denier Trump's ascension to the US presidency has been uppermost on the minds of delegates and negotiators.

Many fear that withdrawal by the United States, a champion of the deal under President Barack Obama, would shatter the political goodwill built up over years of negotiations, and put the very goals of the deal at risk.

Ban joined some 80 heads of state and government for the "high-level segment" of the annual UN climate meeting - the first since last year's adoption of the Paris Agreement to stave off calamitous global warming.

Among the leaders was Sudan's Omar al-Bashir, for whom the International Criminal Court issued an arrest warrant for war crimes, crimes against humanity and genocide in Darfur.

Ban, who welcomed heads of state on the red carpet, was not there to receive Bashir. But the real shadow on the gathering was Trump.

President Francois Hollande of France, which hosted last year's UN climate conference, stressed that the United States "must respect the commitments it has undertaken"."It is not only their duty, it is in their interest as well as (the interest of) all people," he told conference delegates.

The Paris pact sets out the objective of limiting average global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels by cutting planet-heating greenhouse gases from burning coal, oil, and gas.

Ban underlined that no country, "however resourceful or powerful" was immune from the impacts of climate change - rising seas, worsening storms and droughts, spreading diseases and conflict over ever-scarcer resources.

"My sense is that as a very successful business person in the past... I believe that he (Trump) understands that there are market forces already at work on this issue."

Ban was attending the 22nd Conference of Parties (COP22 for short) of the UN's climate convention - his last before his term ends in December.

While waiting for Trump to make his position clear, many are looking to the rest of the world to strongly restate their commitment to the Paris Agreement, with or without the US.

To date, 110 parties have officially ratified the pact, which entered into force on November 4 after crossing the threshold of at least 55 ratifications by countries representing at least 55 percent of greenhouse gas emissions.

They included the US, which represents about 14 percent of emissions. But Trump can withdraw Washington from the deal.

On Tuesday, Ban called for the elimination of fossil fuel subsidies, estimated to exceed $500 billion (465 billion euros) a year, to ease the transition to clean energy.

To bolster the Paris agreement, nations have made non-binding pledges to curb fossil fuel use, but these, collectively, "will not get us out of the danger zone", said Ban.

Last year was the warmest on record, and 2016 seems on course to be hotter still.

"Today more than 150 million people live on land that could be submerged or suffer chronic flooding within this century, possibly causing massive waves of migration and instability," said Ban.

"We have no right to gamble with the fate of future generations or imperil the survival of other species that share our planet."