The European Union has clashed with Barack Obama's administration over climate change over new fears that negotiations on regulating greenhouse gases could break down, according to reports.
The disagreement stems from the way national carbon reduction targets will be counted.
Europe wants to retain systems set up under the current Kyoto Protocol, America wants to replace it with a system of its own, reports the Daily Telegraph.
The US is pushing for each country to set its own emissions rules and decide how to meet targets rather than relying on an international system.
Europe is reportedly reluctant to publicly criticise the Obama administration, partly because he has engaged them in climate change discussion in a way his predecessor did not, writes the newspaper.
But European leaders are allegedly privately worried that America's reluctance could derail the Copenhagen climate summit in December.
The Denmark meeting has been hailed as the last chance to save the planet from a temperature rise of 3.6°F (2°C).