Chinese chief negotiator Su Wei said the 17 per cent reduction in carbon emission offered by the US by 2020 over 2005 levels was neither "remarkable" nor "notable".
UN scientists have underlined the need for aggregate emission reduction by industrialised countries of between minus 25 per cent and 40 per cent over 1990 levels by 2020 with global emissions falling by at least 50 per cent by 2050.
Emphasising that 17 per cent reduction amounted to one per cent reduction on the 1990 level, Wei said: "I doubt if just one per cent reduction can be regarded as remarkable or notable".
"The emissions of the US continue to grow long even though it has long completed industrialisation," he said, pointing out that US emissions grew by 16 per cent from 1990 to 2005.
The Chinese official also criticised Japan as its reduction plan of 25 per cent was conditional on the United States joining the Kyoto Protocol.
But since the US did not intend to be party to the Protocol, Japan's promise did not amount to anything, Wei said.
"The US position is very clear that they will not join the Kyoto Protocol and therefore we find that Japan has committed nothing because they have put something impossible as a condition," he said, pointing out that Japan's emissions grew by eight per cent from 1990 to 2005.
He also said the proposed USD 10 billion fast track fund for funding adaptation activities in poor countries was too little and amounted for roughly USD 2 per person.
"Two US dollars cannot buy a cup of coffee in Denmark," he said.
Noting that developed countries held the "historical responsibility" of reducing emissions and providing finances to developing countries, Wei said the UN Framework Convention on Climate Change stipulates that they reduce their emissions by a large margin in a quantified manner.
Wei also blasted the European Union's targets, saying they were not "ambitious", "meaningful" or "significant".
He said in its first commitment period under the Kyoto Protocol the EU had only reduced its greenhouse gases by eight per cent, that is equal to 2.48 per cent reduction annually
Now they aimed to reduce their emissions by 20 per cent from 1990 levels, he said.
"This is not enough," he said, pointing out that EU reductions would amount to 16 per cent in eight years or 1.05 per cent annually between 2013 and 2020.
"So we can see that the current figure is even less than half of the reduction of the EU in the first commitment period".
Negotiators from 192 countries have come to Copenhagen to hammer out a climate change treaty to succeed the Kyoto Protocol that expires in 2012.