Todd Stern, the US special envoy for climate change, had one aim on arrival in Copenhagen last week: to get tough on China.

He started immediately after stepping off the plane. “I do not envision public funds, certainly not from the US, going to China,” he told his first press conference, dismissing what has been an important Chinese demand for rich countries to fund its efforts to clean up its industrial sector.

The change of tone was stark. For the past year, developed countries have taken pains to praise China lavishly in public. Mr Stern told a conference this year: “China is doing a lot [on cutting emissions], but doesn't get credit for it, and should.”

That was the tactic before the talks, as the US and others sought to bring a wary Beijing to the negotiating table.

But now China is in Copenhagen, with premier Wen Jiabao expected to arrive on Thursday, the gloves are off. Developed countries, in private and in public, are putting the Chinese delegation under intense and co-ordinated pressure.

The harder line has come about partly since developed countries suspect China of using the G77 group of 130 developing nations to advance its agenda at Copenhagen.

China wields great influence among developing countries and is often regarded as the de facto leader. At the talks, China calls press conferences billed “the G77 and China”, but no Chinese representatives turn up and the floor is left to poorer members.

Developing countries argue there is good reason to stick together. “You do not negotiate individually here – you cannot,” said Sergio Serra, Brazil's ambassador for climate change.

But many of the developing countries lining up behind China – such as Sudan and other African nations – have also benefited from Chinese trade deals.

Meles Zenawi, the Ethiopian prime minister and chair of the group of African countries at the talks, is a strong supporter of China, a source of investment and trading partner. At the weekend, he said he had spoken to Mr Wen, who had assured him China would not sign a deal that did not meet African demands for financing by the developed world.

Mr Meles said: “We may need allies in order for us to be heard, allies who have the capacity to mess the environment and therefore allies who could not be ignored.”

China is now one of the world's biggest economies, but Chinese negotiators point out its per capita GDP is relatively low, at about $3,000, and it has 150m people living in poverty. Beijing says it speaks on behalf of developing countries seeking financial assistance even though it does not want western money itself.

When African countries briefly walked out yesterday, forcing a suspension of the talks, it was about an issue China also views as vital: the continuation of the Kyoto protocol.

China and some other developing countries insist “as a matter of principle”, according to He Yafei, Chinese vice-foreign minister, that any successor agreement should continue along the same principles as those of the Kyoto accord.

The 1997 Kyoto protocol divided the world firmly in two: developed countries, which had to accept cuts in emissions; and the rest, with no legally binding commitments.

But the world has changed. For developed countries, it comes down to one inescapable fact: China, the world's biggest emitter of greenhouse gases, is under no international obligation to curb its emissions.

Western countries agree China is still entitled to special treatment. Not even the US asks Beijing to make absolute cuts in emissions within the next decade. Instead, China – in common with the world's other developing countries – is only being required to curb the future growth of its emissions, while rich countries ensure that theirs fall below 1990 levels.