The US threatened to derail a deal on global climate change today in a public showdown with China by expressing deep opposition to the existing Kyoto protocol. The US team also urged other rich countries to join it in setting up a new legal agreement which would, unlike Kyoto, force all countries to reduce emissions.

In a further development, the EU sided strongly with the US in seeking a new agreement, but said that it hoped the best elements of Kyoto could be kept. China and many developing countries immediately hit back stating that the protocol, the world's only legally binding commitment to get countries to reduce emissions, was "not negotiable".

With only a few days of formal UN negotiations remaining before the crunch Copenhagen meeting in December, and the world's two largest emitters refusing to give ground, a third way may now have to be found to secure a climate change agreement. Last night it emerged that lawyers for the EU are in talks with the US delegation urgently seeking a way out of the impasse that now threatens a strong climate deal.

In a day of high international rhetoric, chief US negotiator Jonathan Pershing said the US had moved significantly in the last year. "There has been a startling change in the US position. There is now engagement. We have had a 10-fold increase finance from the US. We have put $80bn into a green economic stimulus package. One year ago there was no commitment to a global agreement."

But he forcefully outlined America's opposition to the Kyoto protocol. "We are not going to be in the Kyoto protocol. We are not going to be part of an agreement that we cannot meet. We say a new agreement has to [be signed] by all countries. Things have changed since Kyoto. Where countries were in 1990 and today is very different. We cannot be stuck with an agreement 20 years old. We want action from all countries."

Yu Qingtai, China's special representative on climate talks, said rich countries should not desert the Kyoto agreement, which all industrialised countries except the US signed up to and was ratified in 2002 after many years of negotiations. It contains no requirement for developing countries to cut greenhouse gas emissions, as both their current and historical emissions are low in most cases. However, China, with its surging economy and rapidly expanding population is now the world's biggest polluter.

"The Kyoto protocol is not negotiable. We want [it] to be strengthened. We don't want to kill Kyoto. We really want a revival, a strengthening of the treaty. That can only be done by Annex I [industrialised] countries having a target of 40% cuts by 2020," said Yu.

"We have an agreement. If you take that away [you remove] the basis of negotiations. There are specific provisions for parties [like the US] who are not signed up to the Kyoto protocol."

China was backed strongly by the G77 group of 130 countries and the Alliance of Small Island States (Aosis), made up of Caribbean and Pacific countries which expect to be made uninhabitable in the next few generations if a strong climate agreement is not secured.

"We face an emergency. We want commitments. We did not create the problem. Any mechanism currently in use is one we want to maintain. National actions are important but they are no substitutes for an international framework," said Dessima Williams, a Grenadian spokeswoman for Aosis.

The EU, today sided openly with the US for the first time. "We look at the Kyoto protocol, but since it came into force we have seen emissions increase. It has not decreased emissions. It's not enough and we need more," said spokesman Karl Falkenberg.

"We are very unlikely to see the US join Kyoto, but we are working with the US to find a legal framework to allow the US to participate and which will allow large emitters [such as China] to participate."

The difference between the sides is now considered to threaten the success of the talks. In essence, the US is insisting on a completely new agreement, with all countries signed up and all countries free to choose and set their own targets and timetable. Most other countries want to keep the existing agreement as a basis for negotiations, to ensure that rich countries are held by international law to agreed cuts. China in particular wants cuts calculated on a per capita basis.

Diplomats last night suggested that the only way out could be for the US to be asked to sign a separate agreement acceptable to developing countries, which would see it cutting emissions at a comparable speed to other countries.

The G77 countries are meeting to consider their oppositions. One diplomat said: "They are very angry. People have talked of walking out."

However, lawyers said it would be difficult to terminate the Kyoto protocol because all parties have to formally agree by consensus to end it. In addition, if no further commitment periods after 2012 are established for rich countries, it would be a breach of their own legal agreements.