China today called on wealthy nations to dramatically increase the rate at which they plan to cut their carbon emissions at international climate negotiations in Tianjin.

The more forthright rhetoric from the hosts broaches a crucial topic that has been notable mainly by its absence at the talks, which began yesterday.

"The emissions reductions goals of developed countries should be dramatically increased," said China's chief negotiator, Su Wei. "We can't discuss other elements and not discuss emissions reductions. It's unavoidable."

Many delegates at this week's working-level negotiations would prefer to leave to a later date this divisive - but fundamental - issue.

Following the disappointment of the Copenhagen climate talks last December, the UN's climate chief, Christiana Figueres, has called on participants this week to focus on achievable goals.

The most likely progress is in the least contentious areas - forestry, finance and technology. If countries can narrow their differences on these issues, they may be able to sign a limited agreement at higher-level talks in Cancún later in the year.

This "balanced package" - as it is vaguely referred to - would include the commitments that countries made last year to reduce greenhouse gas emissions, but critics warn that such a deal can only serve as a band-aid.

"One of the main things here is management of expectations. They are dangerously low," said Li Yan of Greenpeace. "If countries settle for low ambitions, it will become a self-fulfilling prophecy in Cancún."

Scientists and environmental groups warn that existing pledges are far from sufficient to reach the Copenhagen accord goal of keeping global warming within 2C by 2050.

Small island states, which are most at risk from rising sea levels, want to push quickly for a higher target, but in the interests of compromise, they announced in Tianjin that they would be willing to wait until Cancún - or even next year's meeting in South Africa - to secure an agreement.

But patience is limited. China - keen to show leadership among developing nations - said rich countries had to make deeper cuts. "We believe it is a positive thing that they have put forward these targets, but they are certainly still far from the expectations of developing countries and the requirements of science," said Su.

Europe, in turn, wants China to make a bolder long-term commitment about when its own emissions will peak. Jiang Kejun, director of China's leading climate thinktank, the Energy Research Institute, said it would have to come in 2025 if China is to do its share of work in keeping the global temperature rise within two degrees. "That will be technically very difficult for China," he said. "We may have to buy carbon credits." The government has yet to set a peak date.

Jiang, who is relatively optimistic among Chinese advisers, estimates that China's coal production may peak in 2015 at 3.4bn tonnes. But carbon may continue to rise due to gas and oil use. His institute is working on a new study looking at how China might reach the 2025 goal.

"The ball is now in the US court because China is already moving forward with aggressive measures to implement the targets they announced earlier as well as studying more aggressive scenarious," said Barbara Finamore, head of the China programme of the US-based Natural Resources Defense Council. "While China acts, the US is still debating. It's time to move forward."

China's emphasis on carbon reduction targets may be strategic. Any emphasis on emission reductions puts the onus on the United States, which has the world's greatest historic responsibility for greenhouse gases and one of the lowest targets for abating them. US delegates, by contrast, tend to press the issue of transparency, which puts China on the defensive.