"They are the biggest culprit and they are the biggest offender of climate," said Stephan Singer, head of the environmental group WWF's climate change policy unit.

"The United States should take climate change seriously," Singer told reporters in Bangkok, where scientists around the world are attending the week-long session of the Intergovernmental Panel on Climate Change (IPCC), the UN's leading authority on global warming.

While accusing the US of "ignoring science" on global warming, the WWF still urged Washington to lead the world in combating climate change.

"What's happening in the United States is important because it is still the largest emitter" of greenhouse gas, said Hans Verolme, director of WWF's global climate change program.

"The United States should take on economy-wise carbon reduction targets," Verolme said.

The US, the world's biggest economy, consumes around a quarter of global energy and causes nearly 30 percent of worldwide greenhouse gas emissions.

US President George W. Bush has incurred the wrath of environmentalists by abandoning the 1995 Kyoto Protocol, which aims to reduce the emission of climate change-causing carbon dioxide, saying it would hurt the US economy.

Verolme argued not only the US but other top industrialised nations, notably the Group of Eight (G8) countries, should act to find common ground against climate change.

"I think the G8 should show some leadership. They are collectively the world's largest economy, and they should collectively take action," he said, adding climate change was among key topics at next month's G8 summit in Germany.

But Verolme argued a lack of political will from global leaders continued to hamper progress on tackling climate change.

"We can tackle climate change, but that really depends on political action. We need action from all governments," he said.

"Countries do not promote the use of renewable energy. Given current energy prices, I frankly don't know why," the climate change expert said, but adding that it would take some time for the world to see united political leadership.

"Political systems are very slow to change. It's like steering a super tanker."

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