"The majority of the United Nations work still focuses on preventing and ending conflict," Ban said. "But the danger posed by war to all of humanity and to our planet is at least matched by the climate crisis and global warming."

"In coming decades, changes in our environment and the resulting upheavals from droughts to inundated coastal areas to loss of arable land are likely to become a major driver of war and conflict," Ban told an international UN school conference on global warming, meeting in the UN General Assembly hall.

Last month a UN-organized panel of 2,500 top climate scientists from more than 130 nations blamed human activities for global warming and predicted more droughts, heat waves and a slow rise in sea levels that could continue for more than 1,000 years even if greenhouse gas emissions were capped.

The panel predicts a "best estimate" that temperatures would rise by between 1.8 and 4.0 degrees Celsius (3.2 and 7.8 Fahrenheit) in the 21st century.

Ban said the world needed a more coherent system of international environmental governance and that he hoped the United States would take the lead in looking toward the climate change fight beyond Kyoto's end in 2012.

"I hope that United States, while they have taken their role in innovative technologies as well as promoting cleaner energies, will also take the lead in this very important and urgent issue," Ban said.

"INESCAPABLE REALITY"

Ban, who became UN chief on Jan. 1, has pledged to make climate change a top priority and was considering a summit, but his staff said this would not happen. Instead, Ban said, a UN framework conference on climate change will be held in Bali, Indonesia, in December.

"I am encouraged to know that in the industrialized countries from which leadership is most needed, awareness is growing," he said adding that the cost of inaction or delayed action exceeded the short-term investment needed.

The United States is the world's top greenhouse gas emitter and accounts for about a quarter of the global total, ahead of China, Russia and India.

Thirty-five industrialized countries bound by the Kyoto Protocol, which obliges average cuts in emissions of 5 percent below 1990 levels by 2008-12, account for just 30 percent of world emissions.

US President George W. Bush pulled out of Kyoto in 2001, saying that it would damage the US economy and unfairly set no targets for developing nations. But in January he acknowledged climate change as a "serious challenge."

Ban said the success of the Oscar-winning documentary "An Inconvenient Truth," inspired by former US Vice President Al Gore's environmental campaign, showed the issue "is no longer an inconvenient issue, it is an inescapable reality."