Key issues remaining ahead of a major climate summit in Paris this month "have proven too challenging for negotiators to resolve on their own," and responsibility for its success now rests with world leaders, the U.N. secretary-general said Wednesday.

Organizers hope that the summit that begins Nov. 30 will result in the biggest, strongest global agreement ever to fight global warming.

Ban Ki-moon told U.N. member states that 155 countries had submitted their climate plans by the end of October, covering almost 90 percent of global carbon dioxide emissions.

But he said key remaining issues include financial and other support to help developing countries reduce emissions. He called on developed countries to make public pledges of financing before the Paris summit, which runs through Dec. 11.

"This is essential for building the trust that is needed to secure a meaningful, universal agreement.," Ban said.

Developed countries have agreed to boost the flow of climate finance to $100 billion annually by 2020, but hesitate to make firm commitments beyond that. They also want to expand the pool of donors to include China, the world's biggest emitter of climate-warming greenhouse gases, and other emerging economies.

Small islands states and other countries that say their existence is threatened by climate change are warning that a global agreement must be reached. "For us, if we don't get Paris right, it is the beginning of the end," the representative of the Solomon Islands said Wednesday.

Even while U.N. officials note a growing momentum to combat climate change, they say countries' current pledges on cutting emissions won't be enough to keep the rise in global temperatures below 2 degrees Celsius (3.6 degrees Fahrenheit) between pre-industrial times and the end of the century.

Scientists say a temperature rise of more than 2 degrees Celsius could result in profound and irreversible impact on the climate, including flooding of coastal cities and island nations.