Washington says it will not agree to a deal on slashing greenhouse gas emissions at the G8 summit in Germany.
A top US climate official said the G8 should not dictate members' policies, but President Bush said he still had a "strong desire" for a post-Kyoto plan.
Washington's row with Moscow over US plans for a shield in two former Soviet satellites also looms over the summit.
And tensions are also mounting outside the summit area, where police are using water cannon to disperse protesters.
German Chancellor Angela Merkel, hosting the summit, has set what is seen as an ambitious personal goal of persuading the leading industrialised countries to commit to cutting emissions by 50% by 2050.
She also wants them to increase fuel efficiency by 20% and limit the world's temperature rise to 2C.
However, President George W Bush's senior climate adviser said on Wednesday that a number of countries - including the US - would not sign up to those commitments.
James Connaughton insisted that there had been significant progress and consensus on the issue of climate change. But he made clear the US did not believe the G8 should be the forum for setting targets.
"There is significant agreement that those should be established on a national basis, and the only area of disagreement is that the G8 should dictate the national policies of its members," he said.
Mrs Merkel is using the hours before the summit opens to meet the leaders of the Group of Eight club individually.
Speaking ahead of his meeting, Mr Bush said that instead of backing the proposed emissions caps he favoured his own plan that the worst CO2 emitting nations, including the US, China and India, meet before the year is out to put together a new long-term strategy to tackle the problem.
"The United States can serve as a bridge to help find a solution," Mr Bush said.
It is Nicolas Sarkozy's first time representing France at the talks
Anti-poverty campaigners are also hoping for action at this year's G8 summit. On Wednesday evening Mr Bush, accompanied by his wife Laura Bush, will meet Live8 organiser Bob Geldof and U2 front man Bono to discuss poverty in Africa.
BBC diplomatic correspondent James Robbins says meetings of the world's most powerful leaders are always difficult, but this one looks especially tough. Mrs Merkel is going to have to decide how far the most contentious issues can be pushed.
She is operating against a backdrop of increasingly fractious relations between some of the G8 members.
Mr Bush and his Russian counterpart, Vladimir Putin, have been trading threats and accusations ahead of the meeting, sparked by Washington's plans to build a missile defence shield.
The Czech Republic is the planned site for a radar base, while interceptor missiles are due to be deployed in Poland.
Mr Bush, who is expected to meet Mr Putin on the sidelines of the summit, insists that the new system is a "purely defensive measure, aimed not at Russia but at true threats".
However, Mr Putin has scoffed at US claims that the shield is designed to counter threats from states such as North Korea and Iran, and warned that if the US pressed ahead with its plans, Russia would target its weapons at Europe.
On Wednesday Mr Bush dismissed the warning, saying "Russia is not going to attack Europe".
"Russia is not an enemy," he said. "There needs to be no military response because we're not at war with Russia. Russia is not a threat."
UK Prime Minister Tony Blair entered the debate in a BBC interview on Wednesday, saying it was not in Russia's interest to have a "scratchy" relationship with Western countries.
He vowed a "frank discussion" with Mr Putin about his warning.
Protesters equipped with wire cutters have been trying to breach security
Tensions are equally high outside the summit where several hundred anti-globalisation protesters have gathered near the fence protecting the building where the leaders of the eight top industrialised nations are due to meet.
Others are trying to block the roads from the airport, where several delegations have yet to land.
Police are also said to be using tear gas, but there have not been reports of any arrests or injuries.
Some 16,000 officers have been deployed to deal with what are currently estimated to be around 6,000 protesters.
The police are keen to avoid the disturbances seen last weekend in the city of Rostock, where protests turned violent and some 1,000 people were injured.