The 1997 U.N. Kyoto Protocol on global warming aims to reduce greenhouse gas emissions from the developed world by 2012 to 5.2 percent below 1990 levels.

But the United States, the world's biggest air polluter, has refused to ratify the treaty, which it sees as flawed because it does not bind developing countries. It also says it would hurt the U.S. economy.

Waller-Hunter, in New Delhi for a 10-day U.N. climate-change conference of delegates from 185 countries, also said not all countries that had ratified the pact were on course to meet their commitments.

"At the moment, not all parties are on track. Some do better than they're supposed to and some do less than they should do," she said.

Waller-Hunter said it was vital to keep the door open to the United States to eventually draw it into a common framework.

"It's too early to pass judgment because there's a lot of work going on in the U.S. that goes beyond what the federal government committed itself to. We have also to look at initiatives taken by (U.S.) states," she said.

"We should never exclude them (the United States) and hope for a more open attitude towards joining a common framework."

To take effect, the Kyoto pact must be approved by states accounting for at least 55 percent of the industrialised world's 1990 greenhouse gas emissions.

Without the United States, a complex weighting system means the pact would be dead without Russia. But Moscow has backed the treaty and says it may ratify it this year, virtually ensuring its implementation.

The pact, a result of the first Earth Summit in Rio de Janeiro 10 years ago, was signed by then U.S. President Bill Clinton but his successor, George W. Bush, pulled out of the treaty last year.

Waller-Hunter said it was important to meet the pact's objectives to reduce risk of natural disasters, such as floods and droughts which threaten to strike both the developing and the developed world.

Recent climate disasters around the world - from droughts in India and the United States to floods in Europe - have served as graphic harbingers of some of the expected consequences of global warming.

"Temperature increases can range from 1.2 degrees and 5.8 degrees depending on the various scenarios but it's inevitable that some climate change will occur," she said.