Almost 100 world leaders descended on New York as the United Nations (UN) held its summit on climate change on Tuesday, aimed at mobilising political will and strengthening momentum for a fair, effective, and ambitious climate deal in Copenhagen in December.

According to UN Secretary-General Ban Ki-moon, the purpose of the summit was to "focus minds and generate urgent action at the highest levels".

"This year, we have an unprecedented opportunity to prevent climate change from slipping out of control and to jumpstart the transformation to a safer, cleaner, more prosperous green economy that benefits all," he said.

The summit also marked the first UN visit for the Presidents of China and the US, as well as the newly elected Prime Minister (PM) of Japan.

World leaders addressing the opening session following the secretary-general included US President Barack Obama, President of the Republic of Maldives Mohamed Nasheed, Peoples Republic of China President Hu Jintao; Japanese PM Yukio Hatoyama; President of Rwanda Paul Kagame; PM of Sweden Fredrik Reinfeldt; President of Costa Rica Óscar Arias Sánchez; and French President Nicolas Sarkozy.

The World Wide Fund for Nature (WWF) said that leaders meeting at the summit had a historic opportunity to shape a ground-breaking climate agreement that could "help save the world from the ravages of runaway climate change".

The UN emphasised that climate change required strategic focus at the highest political level, as it cuts across all issues - from energy and food security to trade, finance, and international security. Its effects will fundamentally shape the economic growth, development, and security of every country in the world.

"I hope world leaders will leave the summit ready to give their negotiating teams the green light and specific guidance needed to accelerate progress on the road to Copenhagen," said Ban.

"The clock is ticking. I hope they will publicly commit to sealing a deal in Copenhagen," he said.

"Through direct dialogue with their fellow world leaders, we hope summit participants will come to grips with the urgent need for action, as well as the benefits of transforming their economies along a cleaner energy, low-emissions pathway," Ban added.

"Negotiations for the UN climate agreement have proceeded at a snail's pace this year, stalled by a lack of clear political leadership," said WWF International DG Jim Leape.

"Climate change is the foremost threat to our environment, our economy and our security. We will succeed in meeting this challenge only if the heads of state now gathering in New York commit themselves to bold action to reduce emissions, and to support those suffering already from climate change impacts," he added.

Although not a negotiating session, the summit would seek to find a common understanding among the leaders on a path forward. This understanding would be captured in a chairman's summary presented by Ban at the conclusion of the summit.

Intergovernmental Panel on Climate Change (IPCC) chair Dr Rajendra Pachauri would present the scientific basis for action. Professor Wangari Maathai, recognised for her tree planting and environmental efforts among the poorest communities, would speak for civil society.

Former US vice president Al Gore, who was a co-winner of the Nobel Prize with the IPCC, would address the UN leadership forum luncheon. This forum marked the first time that world leaders and leaders from the business, investor, and civil society communities would meet on such a large scale at the UN.

"Leaders of wealthy nations have promised to limit warming to below 2 °C but have not yet committed to anything like the required level of emissions reductions. The UN Summit is the place to recognise that what's on the table now is not enough and that developed countries need to come back with more before Copenhagen," said Leape.

"At the same time, leaders of major developing countries must make their commitment to climate action more visible to the world. Countries like China, India, Mexico and South Africa are already working very actively to combat climate change, but we still need to see them come forward and put these actions into a global climate framework," emphasised Leape.

The UN would offset participants' travel by purchasing carbon emission credits from the Clean Development Mechanism, a tool that directs investment in clean technologies to developing countries.

The summit's emissions would be offset by investments in a project in a poor semi-arid region in Kolar District, Karnataka State, India, seeking to reduce deforestation and provide clean energy for cooking.