As the United States lags on climate legislation, China is poised to join the European Union in claiming "front-runner" status among nations battling climate change, the UN climate chief said Monday.

Yvo de Boer said in an Associated Press interview that China is leaping ahead of the United States with domestic plans for more energy efficiency, renewable sources of power, cuts in vehicle pollution and closures of dirty plants.

The development marks a dramatic turnabout. The United States, under former President George W. Bush’s administration, long cited inaction by China and India as the reason for rejecting mandatory cuts in greenhouse gases.

"China and India have announced very ambitious national climate change plans. In the case of China, so ambitious that it could well become the front-runner in the fight to address climate change," de Boer said. "The big question mark is the US."

He spoke on the eve of a UN summit of 100 world leaders intended to rally momentum for crafting a new global climate pact at Copenhagen, Denmark in December. Bush had rejected the 1997 Kyoto Protocol for cutting global emissions of warming gases based on its exclusion of major developing nations like China and India.

Chinese President Hu Jintao will announce new plans to fight global warming at a UN summit on climate change on Tuesday. China already has said it is seeking to use 15 percent of its energy from renewable sources by 2020.

China and the US together account for about 40 percent of all the world’s emissions of carbon dioxide, methane and other industrial warming gases.

De Boer said he also was encouraged by Japan’s new goal of a 25 percent cut in greenhouse gas emissions from 1990 levels by 2020.