The EU and scores of developing countries have pledged to toughen their existing commitments to reduce greenhouse gas emissions to enable the world to stay within a 1.5C rise in global warming.

The promise, which follows increasingly dire scientific warnings, was the most positive message yet to come from the ongoing talks in Poland.

The announcement came at the end of a day in which the UN secretary general made an impassioned intervention to rescue the talks, which have been distracted by US, Russian and Saudi moves to downgrade scientific advice.

“We’re running out of time,” António Guterres told the plenary. “To waste this opportunity would compromise our last best chance to stop runaway climate change. It would not only be immoral, it would be suicidal.”

The talks have centred on devising a rulebook for implementing the 2015 Paris agreement and raising countries’ level of ambition to counter climate change, but progress has been slow on several key issues and divisions have emerged between four fossil fuel powers – the US, Russia, Saudi Arabia and Kuwait – and the rest of the world.

The UN believes China could play a stronger role in the absence of leadership from the US. Sources said Guterres would make a telephone call to Xi to ask for his help in nudging talks forward.

The EU also wants China, which is a key member of the block of 77 developing countries, to step up to ensure that countries all follow the same rules in being transparent over their greenhouse gas emissions.

Campaigners praised the decision by the High Ambition Coalition group of countries, made up of the EU and four other developed countries, including Canada and New Zealand, as well as the large grouping of least developed countries and several other developing nations, to scale up their emissions-cutting efforts in line with a 1.5C temperature rise limit.

Wendel Trio, director of the Climate Action Network Europe, said: “The spirit of Paris is back. The statement will boost greater ambition at the crunch time of these so far underwhelming talks. For the EU this must mean a commitment to significantly increase its 2030 target by 2020, even beyond the 55% reduction some member states and the European parliament are calling for. We call upon the countries that have not signed the statement so far to stop ignoring the science.”

Guterres, in a pointed criticism aimed at the four countries that have been refusing to “welcome” the Intergovernmental Panel on Climate Change’s special report on 1.5-degree warming, said rejecting climate science was indefensible.

He added: “The IPCC special report is a stark acknowledgment of what the consequences of global warming beyond 1.5 degrees will mean for billions of people around the world, especially those who call small island states home. This is not good news, but we cannot afford to ignore it.”

Frank Bainimarama, the prime minister of Fiji and the outgoing chair of COP23, amplified Guterres’ message. He told delegates they risked going down in history as “the generation that blew it – that sacrificed the health of our world and ultimately betrayed humanity because we didn’t have the courage and foresight to go beyond our short-term individual concerns: craven, irresponsible and selfish”.

Al Gore at the COP24 UN climate change conference 2018 in Katowice, Poland. Photograph: Agencja Gazeta/Reuters

The former US vice-president Al Gore told delegates they faced “the single most important moral choice in history of humanity”.

Behind the scenes, delegates said there had been strong progress on finance thanks to a doubling of commitments by Germany and Norway to help poorer nations adapt to climate change and build institutions capable of monitoring emissions. Nicholas Stern, the author of a landmark review on the economics of climate change, praised “the level of ideas and cooperation”.

But others said there were still many disputed brackets in the negotiating text on transparency and other elements of the rulebook.

“There has been some progress, but it’s a very worrying time. There is still a lot more on the table than we hoped for at this stage,” said Helen Mountford, vice-president of the World Resources Institute. “The secretary general is coming in to make sure this COP can land in a good place. He will hold a summit next year to raise ambitions. If he wants success there, then here we need a robust rulebook and clear signals on ambition and finance.”

Janos Pasztor, the former climate adviser to Ban Ki-moon, told the Guardian that Guterres was doing the right thing by intervening at a crucial stage. “He needs to make clear what the IPCC has described as a major challenge, and that we have to deliver on that,” he said.

Pasztor added: “We are talking about the need for massive emissions reductions, that have to happen now, not in the future. It is very daunting. The secretary general has reminded the world of what is at stake, and the political significance of that.”

The contrasts with the Paris climate summit, in terms of the political atmosphere, were striking. David Levaï, who was part of the French government team that helped to broker the successful 2015 conference, said the geopolitical winds were far less favourable today. Globally, the rise of nationalists such as Donald Trump in the US and Jair Bolsonaro in Brazil has tilted power towards fossil fuel and agribusiness interests.

He said: “In the year before Paris, all countries made clear that they wanted an agreement. Now, there are repeated attacks on multilateralism, and this has empowered groups that take negative actions.”

Levaï, who is at the Institute for Sustainable Development and International Relations, expressed hope that the secretary general might make a difference. “The fact that he has come back shows he feels a need to whip countries into order,” he said.

There has been some criticism of the pro-coal government of Poland for failing to press governments to raise their ambitions. But it joined Fiji as co-chair of the Talanoa phase of the negotiations to issue a call for action that recognised the importance of the 1.5C report as the basis for more urgency and ambition.

“The window for action is closing fast. We need to do more and we need to do it now,” said the document, which would form part of the official statement from this conference.