Tension, tiredness and reports of lack of progress on the issues that have repeatedly surfaced at the UN climate talks were all too evident on Thursday as negotiators attempted to forge a global deal on the Paris Agreement rulebook.

China accused some of the richest nations in the world of “backsliding” on promises to clean up pollution, reduce greenhouse gas emissions, and provide $100 billion a year in climate-related aid by 2020.

Poland spearheaded a push by a group of the 79 least developed countries for clarity on when those promises will be fulfilled. It indicated deepening rifts as the talks that were due to conclude in Katowice on Friday but are likely to run into the weekend.

“Some delegations we are seeing backsliding,” Xie Zhenhua, China’s lead envoy to the talks in Katowice, said at a briefing on Thursday afternoon. “There are still quite a number of developed countries who did not start” providing financial and technological support that they had pledged in the Paris Agreement three years ago. “We cannot accept any backsliding.”

The remarks illustrated the divide on environmental issues between China and the US, which had worked together to seal the Paris pact in 2015.

The NGO ActionAid warned that ambition on averting catastrophic climate change “is at risk of fizzling out”.

“Ministers are here to agree the rulebook needed to implement the 2015 Paris Agreement to limit global warming to 1.5 degrees and protect the planet now and for future generations,” said Harjeet Singh, its lead on climate change. “The alternative is unconscionable: Up to 150 million more premature deaths and millions more people forced to migrate because of climate change.”

A coalition of African environmental and climate justice groups said the report was “under contemptuous attack by a coalition of climate deniers revolving around the US”.

What’s worrying many delegates and observers is the complexity of the task of delivering a strong rulebook. That concern was echoed by Prof John Sweeney who is representing An Taisce at COP24.

The Polish presidency, which is chairing the talks, has assumed greater control of proceedings and was due to issue a final text on Thursday night.

“I think it’s pretty normal for the Thursday [before the final day] but suspect there is more resistance to compromise this year,” Prof Sweeney told The Irish Times. There is a notable lack of clarity on emissions targets and how they are accounted for, he added. Without a level playing field, there was a risk some countries would exploit ambiguities.

Developing nations were resisting measures that would ensure transparency in the way emission cuts are measured, reported and verified.

There has also been an attempt by a small number of countries to remove human rights references from the pre-amble of the Paris agreement. Former president and climate justice campaigner Mary Robinson fought hard to get it more centrally featured.

What might not be the final day will coincide with more school strikes across the world, notably in the US and Germany, as young people demonstrate concern about a lack on progress in addressing climate change.

Decades of efforts to curb climate-changing emissions were failing, leaving not just small islands but “every country” at this year’s climate talks facing the threat of disaster, former Maldives president Mohamed Nasheed warned.

UK Energy and Clean Growth Minister Claire Perry confirmed the UK has written to the UN offering to host the 2020 climate summit, which would coincide with the Paris pact coming into full effect.

It would also come just weeks after the next US election, which is likely to see any Democrat candidate run on a ticket of returning the US to the accord, while President Trump will seek to rubber-stamp his decision to quit the treaty. The day after the election, the US must affirm its withdrawal or not.