World leaders on Tuesday urged ambitious action to combat climate change, but pledges remained well short of goals, with a year to go for an accord.

At a UN summit held after tens of thousands rallied around the world, France promised $1 billion to the Green Climate Fund -- making it the only contributor other than Germany to the new institution that would help the worst-hit countries.

But UN Secretary-General Ban Ki-moon called the meeting more to build momentum than to reach concrete achievements. It was the first such event since the Copenhagen summit on climate change ended in disarray in 2009 and aims to set the tone for a conference next year in Paris designed to seal a new global agreement.

French President Francois Hollande said the Paris conference should deliver a "global and ambitious" deal and warned that climate change posed a "threat to world peace and security."

US President Barack Obama, addressing the summit hours after ordering strikes on Syria, said that the "urgent and growing threat of climate change" would ultimately "define the contours of this century more dramatically than any other" issue.

"We know what we have to do to avoid irreparable harm. We have to cut carbon pollution in our own countries to prevent the worst effects of climate change," Obama said.

Obama called for an "ambitious" but also "flexible" agreement -- a nod to political difficulties he would face if he needed the US Congress to ratify a treaty.

Developing nations have balked at signing on to a binding accord without firm US commitments, noting that wealthy countries bear historic responsibility for climate change.

Obama said he met in New York with Chinese Vice Premier Zhang Gaoli -- President Xi Jinping was notably absent -- and told him that the world's two largest economies "have a special responsibility to lead" on climate change.

Zhang said that China, which has surpassed the United States as the world's top carbon polluter, wanted its emissions to peak as soon as possible.

He was not more specific but senior official Xie Zhenhua told reporters that China was making good on reductions in its carbon intensity and would announce early next year a prediction of when its emissions would peak.

- 'Vague promises' -

UN chief Ban Ki-moon said that $2.3 billion has been pledged to the Green Climate Fund, but the amount was well below the $10 billion aimed for by year-end -- let alone the $100 billion a year aimed starting in 2020 to help poor nations cope with rising temperatures and disasters.

With France and Germany accounting for $2 billion of the amount, Tim Gore, head of climate policy for aid group Oxfam, criticized the "partial and piecemeal response" to climate change from donors.

Activist movement ActionAid said the summit offered just "vague promises" on climate change despite the unprecedented international demonstrations on Sunday that urged immediate action.

"We welcome new pledges of money from France and others, but they fall well short of what's needed," said ActionAid USA's Brandon Wu.

Small island nations fear that climate change will literally wipe them out as water levels rise, turning their people into environmental refugees.

"Tuvalu's future is in your hands. The time for denial, for hesitation, for pandering to the interests of the fossil fuel industry is over," said the Pacific island's prime minister, Enele Sosene Sopoaga.

- No longer 'fiction' -

In one notable promise, New York Mayor Bill de Blasio vowed that the metropolis -- and host of the UN -- would cut its emissions by 80 percent by 2050, a level of ambition that activists say is needed at a national level.

Nations including the Democratic Republic of Congo, Ethiopia, Guatemala and Uganda said they would restore 30 million hectares (75 million acres) of tropical forest, more than doubling the 20 million hectares already pledged through a global initiative.

Leonardo DiCaprio brought star power to the UN summit, urging leaders to stop treating global warming as if it were a fiction.

"As an actor, I pretend for a living. I play fictitious characters often solving fictitious problems," DiCaprio, sporting a ponytail and suit and tie, told the summit.

"I believe mankind has looked at climate change in that same way -- as if it were fiction."