"The climate efforts do not stop at 2020," said Swedish Prime Minister Fredrik Reinfeldt, whose country holds the EU's rotating presidency until the end of the year.

"It's correct also to have ideas and discussions also for what happens after 2020," he told reporters at the European parliament, before heading to Copenhagen with European Commission chief Jose Manuel Barroso.

The underlying aim of efforts to fight climate change is to limit the average rise in world temperatures to no more than two degrees Celsius above pre-industrial levels.

As part of the effort to achieve that, the EU has pledged to cut carbon dioxide emissions by 20 percent by 2020, compared to 1990 levels. It will deepen that cut to 30 percent if other major polluters join the effort.

But Reinfeldt said that not all countries are using a level playing field to calculate their commitments to cut emissions of carbon dioxide -- the main greenhouse gas -- and predicted they would struggle to match the EU offer.

"The United States starts at 2005, so they have another base year," he said.

"We don't have the same definition. The important question is what happened between 1990 and 2005 in the United States? Well they have increased of course their emissions by 18 percent," he went on. "This is an important question."

He suggested the United States could not match the offer from Europe, which prides itself as leader of the fight against global warming.

"We are still putting on the table the obligation to go to 30 percent already to 2020 if other developed countries would make the same kind of contribution," he said.

"But of course that is asking of other developed countries, most importantly Canada and the United States, to make an increased engagement, increased figures, so that we could say they are also in line with our contribution.

"That is not the fact today," he said.

On Tuesday, the world's two biggest carbon emitters, China and the United States, said they would not shift on their emissions pledges, the thorniest problem plaguing the climate talks in Copenhagen.

US President Barack Obama has offered to cut US carbon emissions by 17 percent by 2020 over the 2005 benchmark, a figure that aligns with legislation put before the US Congress.

But that amounts to a reduction of around four percent compared with the more widely used reference year of 1990.

Greens party president Rebecca Harms warned that the EU would undermine the talks if it steps back from its 30 percent offer, saying talk of a change was "extremely worrying at such a crucial stage in the negotiations".

"The EU has waved this promise under the noses of all other negotiating parties for two years: to revoke it now could jeopardise the talks irreparably," she said in a statement.

But Barroso told parliament that the EU was looking to set future benchmarks for climate negotiations with the rest of the world.

"I have mentioned the possibility of having some modulation in our offer, namely the possibility of constructing some passways beyond 2020," he said. "This negotiation is not just about 2020, it's after 2020."

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