UN members launched a new round of climate talks Wednesday as Europe demands early action to tame carbon emissions, two days after the United States unveiled a longer-term plan.

"Although we are already looking beyond the current decade, it is also crucial to step up action before 2020," the European Union's climate commissioner, Connie Hedegaard, said.

The EU is on track to reduce emissions of greenhouse gases in 2020 by about 24 percent over 1990 levels, more than its targeted cut of 20 percent, she said.

The "overachievement" amounts to a saving of some 5.5 billion tonnes of carbon overall, Hedegaard said.

"We are making a significant contribution to closing the 'ambition gap' between what the world needs to do and what countries intend to do by the end of this decade," she said.

"We ask other major economies to come forward with concrete ways to step up their ambition."

The 12-day session under the UN Framework Convention on Climate Change (UNFCCC) aims at clearing some roadblocks to a post-2020 global pact on dangerous man-made carbon emissions.

The Bonn meeting -- to be attended by several dozen ministers on Thursday and Friday -- is also supposed to ramp up pledges for tackling emissions before 2020.

In March, the UN's top scientific panel warned that action in the next few years may dictate whether the target of limiting warming to two degrees Celsius (3.6 degrees Fahrenheit) will be met easily and affordably, or at greater economic pain and human cost.

Key components of post-2020 and pre-2020 action are to be fleshed out in Lima in December, preceded in September by a special summit at the UN in New York hosted by Secretary General Ban Ki-moon.

On Monday, US President Barack Obama unveiled his most ambitious action yet on climate change.

He proposed ordering cuts of up to 30 percent in emissions from power plants by 2030 compared to levels in 2005.

Analysts said the proposal, if implemented, would help the United States join the EU and China in meeting pledges they made in 2010.

But, they warned, the new global deal due to be signed in Paris next year, would have to deliver much deeper cuts to meet the 2 C target.

The UN's Intergovernmental Panel on Climate Change (IPCC) warns that by 2100 the planet could be up to 4.8 C warmer and sea levels 26-82 centimetres (10-32 inches) higher.

Conflict, hunger, floods and mass displacement from coastal erosion could be the likely result, it says.