US Chief negotiator Todd Stern, who arrived on Wednesday, said the US is committed to getting the "strongest political agreement" at Copenhagen for which "a real commitment from China" was a major necessity.

Stern noted that developing countries including China, India and South Africa had announced "significant" proposals to cut emissions domestically but said these would have to be "wrapped up" in an international agreement.

"What is important if we have to get an international agreement is not that people announce things domestically but they put them in an international agreement," he said.

"It becomes part of the international agreement and not just a press release domestically".

Negotiators at the 192-nation U.N. conference in Copenhagen are working to bridge the chasm between rich and poor countries over how to share the burden of fighting climate change.

While the US had the largest historic role in greenhouse gas emissions but most of the emissions now going forward would be from the developing countries, he said.

"There is no way to solve this problem by giving the major developing economies a pass," he said, making it clear that a transparent commitment would not be required from the vast number of developing countries but only the major ones.

China is the world's largest emitter of greenhouse gases while India is the fifth largest.

"You can't even think about controlling this problem without a real commitment from China," Stern said.

Developing countries, including India and China, have maintained that that given the fact that emissions from industrialising developed nations over the last century have been the primary cause of global warming, they should shoulder greater responsibility for carbon cuts.

Stern quoted the International Energy Agency estimates that 97 per cent of emissions between now and 2030 will come from developing countries and 50 per cent from China alone.

He said putting mitigation commitments in a legally binding agreement was essential for "transparency" in the international community, which could then gauge how the crisis was being resolved.

"We can't be in a world where transparency is just trust," he said.

"There needs to be transparency so that everyone can have confidence that everybody else is undertaking what they said they would do".

The US top negotiator also stated that while US wanted a legally binding treaty, it would back a "politically binding agreement" if states failed to commit to a legal document.

"There is an advantage that an agreement (politically binding) can take effect right away," he said, adding that politically binding was no substitute to a legally binding agreement in the near future.

The US negotiator vehemently rejected the notion that developed countries had a "historical debt," from 200 years of pollution during their period of industrialisation.

"I completely reject the notion of the debt or of reparations," Stern said.

People at that time were "blissfully ignorant" that they were pumping green house gases into the atmosphere, he said.

"We absolutely recognise the historic role in putting emissions in the atmosphere but a sense of guilt or culpability I categorically reject that".

The Climate Change envoy also reiterated that the US would not become a party to the Kyoto Protocol.

He, however, said the US would support the fast track funding of USD 10 billion that was part of the Commonwealth Declaration, which will be aimed at the poorest countries.