UN Secretary-General Antonio Guterres on Friday said a $100 billion annual fund to help developing countries tackle climate change was crucial to achieve the goals of the Paris climate deal.
"We need to mobilize the agreed $100 billion annually for developing countries," Guterres told reporters ahead of his trip to Bonn to attend the UN climate conference.
"This is crucial to spur action and to build trust. We are far from having that entirely guaranteed."
Under the Paris climate deal, wealthy countries renewed their commitment to raise $100 billion by 2020 to help developing countries deal with global warming, but there are concerns that they will not reach that target.
In June, President Donald Trump said the United States would no longer contribute to the Green Climate Fund, which helps poorer countries adapt to climate change, when he announced the US withdrawal from the Paris deal.
Guterres said carbon pricing was an "extremely important instrument that must be developed" to cut emissions and fulfill the goals of the Paris agreement.
UN negotiators meeting in Bonn are trying to work out how to implement the Paris accord, which aims to keep warming at "well under two degrees Celsius" (3.6 degrees Fahrenheit) above pre-industrial levels.
Guterres will deliver an address at the conference on Wednesday which he said was taking place at a pivotal moment.
The World Meteorological Organization said in an alarming report last month that the concentration of carbon dioxide in the atmosphere had hit its highest level in 800,000 years.
"We need to accelerate climate action -- and we need to raise ambition," said Guterres.
The window of opportunity to meet the 2-degree target may close in 20 years or less, he said, adding that there must be at least a further 25 percent cut in emissions by 2020.
China is the world's top carbon polluter, followed by the United States, the European Union, India and Russia.
A report this week said wealthy countries were falling well short of their pledge to provide the $100 billion annually.
Of the $111 billion invested in clean energy technologies only $10 billion was provided by rich countries, according to the study by Bloomberg New Energy Finance.