The comments, by top Group of Eight (G-8) envoy Bernd Pfaffenbach, suggest Merkel faces a daunting diplomatic task over the coming weeks to convince Washington to back a climate deal she hopes will be the centrepiece of her G-8 presidency.

Given at a briefing to a small group of reporters in Berlin, the remarks appeared intended to dampen expectations of a breakthrough at the June 6-8 summit in the Baltic resort of Heiligendamm and to raise pressure on the White House ahead of a Monday meeting between Merkel and US President George W Bush.

"It is proving to be one of my toughest areas in the negotiations," Pfaffenbach said of the climate change issue.

"This is partly because of ... the reservations, if not resistance, from one particularly big country," he added, expressing frustration with the pre-summit talks.

While Pfaffenbach took care not to mention the United States by name, it was widely understood that Washington was his intended target. The Bush administration, which did not sign up to the Kyoto Protocol on climate change, has long been reluctant to make a commitment to curb the greenhouse gases blamed for swelling sea levels and causing droughts as well as floods. Merkel, who holds the rotating presidencies of the G8 and European Union, has been hoping her close personal relationship with Bush and mounting public awareness of the risks of global warming will help her win over the United States.

Germany has also invited non-G8 members Brazil, China, India, Mexico and South Africa to the summit, ensuring countries producing close to 90 percent of global emissions are there.

Pfaffenbach pointed to a deal brokered by Merkel at a summit in Brussels last month, which commits the 27-nation European Union to reduce carbon dioxide emissions by 20% by 2020 and boost renewable energy sources.

"She wants to internationalise as much as possible of this deal," Pfaffenbach said.

Merkel is due to meet Bush in Washington on Monday to sign an EU-US deal to harmonise regulatory standards and remove non-tariff barriers to trade.

The leaders are also expected to discuss energy and the environment, although diplomats say Washington is resisting an EU push for a strong joint statement on climate change.

Joachim Wuermeling, a senior official in the German Economy Ministry in charge of energy issues, told Reuters in an interview this weekend that Germany wanted G8 countries to make binding commitments to boost energy efficiency.

Two years ago, British Prime Minister Tony Blair failed to forge an international consensus to combat global warming at a G8 summit in Scotland amid resistance from Bush.

—Reuters