The White House said all key polluting nations would have to be involved in any long-term agreement on climate change.

"We've not sat down with China, India, Brazil, Mexico, South Africa," Jim Connaughton, chairman of the US administration's Council on Environmental Quality, said ahead of talks between Merkel and US President George W. Bush.

"We have not sat down with Australia, South Korea and a number of the other major emitting countries on this issue and so until we've got everyone in the room and until we have consensus among all of them, you won't see a collectively stated goal on that yet but it's coming."

Bush insisted after meeting with Merkel on his good will to eventually striking a deal.

"I come with a strong desire to work with you on a post-Kyoto agreement and about how we can achieve major objectives," Bush said, referring to the UN-backed Kyoto Protocol on capping emissions that expires in 2012.

"One of those of course is the reduction of greenhouse gases, another is becoming more energy independent, in our case from crude oil."

Merkel put a brave face on the impasse, calling her talks with Bush "very good and successful."

"I hope that we can join together to send a strong message," she said.

Merkel has staked her Group of Eight presidency on pushing for agreement to limit the global temperature rise to two degrees Celsius (3.6 degrees Fahrenheit) and cap carbon emissions by 50 percent compared with 1990 levels by 2050.

The German plan has won qualified support from some G8 nations, but Merkel has faced stiff US opposition over her call for mandatory emissions limits.

She wants the G8 to show leadership ahead of negotiations in Bali, Indonesia, in December to find a successor to Kyoto.

In the face of strong resistance to a deal, the daily Frankfurter Allgemeine Zeitung reported that Merkel preferred to let the summit fail than agree to a watered-down climate pact.

The newspaper cited Berlin sources as saying that Merkel would reject a "coalition of the willing," with select countries agreeing to cut emissions, and insist on winning an accord under a UN framework.

Environmental watchdog Greenpeace urged her not to buckle, saying anything less than a US agreement to ratify the Kyoto Protocol and accept the German targets was simply "hot air."

With the major players at loggerheads, Japanese Prime Minister Shinzo Abe offered to act as a go-between in the fraught debate.

He said with Europe and the United States divided over their positions on climate change, "Japan has to take the initiative to lead all the countries in one direction that they can basically accept. I feel that is our important responsibility."

Facing increasing international pressure, Bush last week unveiled a climate plan for the United States and up to 14 other big emitters to agree by the end of next year including "a long-term global goal" for reducing greenhouse gases.

Although he welcomed US movement on the issue, Bush's key ally British Prime Minister Tony Blair backed Merkel, saying anything less than a global deal on significant carbon emissions cuts would be a failure.

He told the Guardian daily that the "important thing is that if we get an agreement to the idea of a global target of a substantial reduction in emissions, and it needs to be clear that it is in the order of 50 percent. You are not talking about 20 percent."

Meanwhile new French President Nicolas Sarkozy called Bush's proposal "not sufficient," in an interview with The New York Times.

The major emerging powers who have been invited to the G8 summit have also expressed reservations about binding targets.

Indian Prime Minister Manmohan Singh said that "due care must be taken not to allow growth and development prospects in the developing world to be undermined or constrained."

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