"It is clear that we need a more ambitious position on the part of the United States," Barroso told Friday's edition of the Financial Times Deutschland newspaper.

"The United States are putting a lot of emphasis on market mechanisms in the fight against climate change, and they are right to do so," Barroso said.

"But market mechanisms only work if there are binding targets.

"As a major emitter of toxic gases, the United States naturally bears a particular responsibility."

Barroso said he did not believe that Bush's promise to work with his fellow leaders of the Group of Eight wealthiest nations to create a new framework to cap greenhouse gas emissions made it more likely that a commitment on the issue would be struck at the summit in the northern German resort of Heiligendamm.

"I hope that the United States intends to use the meeting as an opportunity to make the G8 summit contribute towards the UN's multilateral climate protection system," Barroso said.

The international community is due to gather in Bali in December to begin negotiations to replace the UN-backed Kyoto Protocol on limiting greenhouse gases which expires in 2012.

Barroso said: "In the US Congress there is very visible support for more ambitious proposals. It is all just a question of time.

"I hope we will make a genuine breakthrough in 2009 for the post-Kyoto era."

In a speech on Thursday, Bush proposed that "by the end of next year, America and other nations will set a long-term global goal for reducing greenhouse gases" in consultation with major greenhouse gas-producing nations, including fast-growing China and India.

Merkel has staked Germany's year-long G8 presidency on finding a binding agreement between the leading industrial powers on limiting the emission of greenhouse gases.

The United States has argued that it favours a technological approach to fighting global warming rather than imposing emissions limits which it fears could hamper economic growth.

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