German Chancellor Angela Merkel is in last-ditch efforts to secure climate change goals at a G8 summit marred by a row over US missile defence plans.
The US has already said it and several other countries at the summit of top industrial states oppose her target. At the same time, US plans to build a missile shield in two former Soviet satellites have sparked an intense war of words between Moscow and Washington.
Mr Putin warns Russia will aim weapons at Europe if the US pursues the plan.
UK Prime Minister Tony Blair says there must be a "frank discussion" about this threat behind the scenes at the summit, which is taking place in the German Baltic resort of Heiligendamm.
Keeping cool
Mrs Merkel 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.
Mrs Merkel is due to meet Mr Bush and every other leader of the Group of Eight club individually before the summit opens.
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.
'Don't worry
She is operating against a backdrop of increasingly fractious relations between some of the G8 members.
US President George W 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, said the new system was a "purely defensive measure, aimed not at Russia but at true threats". Speaking in the Czech Republic on Tuesday, the American president said the Cold War was over and Russia was not an enemy of the US. But he went on to accuse Russia of sliding backwards on democracy.
"Reforms that once promised to empower citizens have been derailed, with troubling implications for democratic development," he said.
For his part Mr Putin has scoffed at US claims that the shield is designed to counter threats from states such as North Korea and Iran, indicating that the real target was Russia.
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.
London currently has its own issues with Moscow over the murder of former KGB agent Alexander Litvinenko.
Mr Putin has dismissed a UK extradition request for a Russian suspect as "pure foolishness".
Pushing at frontiers
Other issues on the agenda of the summit include the Middle East and the future of Kosovo.
Meanwhile, the German authorities are on alert to prevent disturbances at the summit. As Mr Bush arrived at Rostock in Germany, hundreds of protesters gathered near the airport shouting: "President Bush is not welcome here."
President Bush was quickly taken by helicopter to the summit venue.
The crowd soon dispersed and the latest demonstration passed off peacefully.
During the weekend German riot police clashed with anti-globalisation protesters in the city. About 1,000 people were injured.
(BBC News, 06/06/2007)