Fighting global warming will get tougher once the Kyoto protocol ends in 2012 and the world must try to get Washington involved in the long term, the head of the U.N. Environment Program said on Wednesday.

Klaus Toepfer also welcomed Russia's ratification of the 128-nation plan, triggering a countdown to Kyoto's entry into force on February 16 next year even though the United States pulled out in 2001.

He said countries should already start planning for what he said would be harder, deeper curbs on emissions of heat-trapping gases after Kyoto. The pact aims to cut emissions by developed nations by 5.2 percent below 1990 levels by 2008-12.

"We have to discuss beyond Kyoto, we have to start the negotiations," he told Reuters on the sidelines of an eight-nation conference on a rapid thaw of the Arctic region.

He said Kyoto cuts could be easily reached like "low-hanging fruits" but that beyond Kyoto "you have really to ask about the energy structure in future, how do you change the carbon intensity, how to increase efficiency."

U.N. scientists say that gases from burning fossil fuels in cars, power plants and factories are blanketing the planet and driving up temperatures. That could cause catastrophic floods, storms and raise sea levels by almost 3 feet by 2100.

He said U.S. involvement after 2012 was a key to encouraging a wider assault on global warming, especially in China or India which have fast-growing economies and account for 40 percent of the world's population.

"From 2012 I believe that beyond Kyoto we should have a very, very central target of (enlisting) the United States," he said.

NO PROMISES

But Washington is making no promises. President Bush (news - web sites) has said Kyoto would cost too much and wrongly excludes poor states, which have no targets for curbs until 2012.

"Right now we think it's necessary to focus on the (domestic) plans we have put in place," said Paula Dobriansky, U.S. Under Secretary, Global Affairs, when asked if Washington might take part in global planning for beyond 2012.

Washington aims to cut the amount of greenhouse gases emitted for every dollar of U.S. economic output by 18 percent over the decade to 2012. The policy will brake a rise in emissions but stops short of caps sought by Kyoto.

"In 2012 we will take stock of the trends and then we will reassess our approach if necessary," Dobriansky told Reuters.

Toepfer, a German, said that the United States was the top emitter of carbon dioxide, the main greenhouse gas, but also a key source of new technology for non-polluting energies like solar or wind power.

Toepfer praised British Prime Minister Tony Blair (news - web sites) for taking a lead and planning a 60 percent cut in greenhouse gas emissions by 2050. But he said it would mean wrenching change.

"I don't want to water it down but (Kyoto targets) were the low-hanging fruits," he said. "You cannot get to 60 percent with low-hanging fruits."

Kyoto is meant to be a first step against global warming. U.N. projections show that Kyoto will brake rising world temperatures by only about 0.1C (0.2F), a pinprick compared to a forecast rise of 1.4-5.8C by 2100.