Battle lines have been exposed amid intense diplomatic wrangling as crucial United Nations climate talks reach the end game.
Ministers from more than 190 countries were gearing up to work through the night again at the summit in Paris to secure a final draft of the deal as the talks slipped past their official Friday evening deadline.
The final deal could include references to making efforts to keep temperature rises to no more than 1.5C above pre-industrial levels - a key ask of the most vulnerable countries who fear greater warming could threaten their very survival - as many countries are thought to be "comfortable" with the idea.
Although it could still change in the final draft, as it stands the agreement would seek to keep temperature rises to "well below" 2C above pre-industrial levels - beyond which dangerous climate change is widely expected - as well as to pursue efforts to limit the temperature increase to 1.5C.
But there are warnings about the feasibility of meeting the 1.5C target, with Professor Geoffrey Maitland, professor of energy engineering at Imperial College London, describing it as "extremely challenging, verging on the unrealistic" given that global temperature rises are already at 1C.
"However, if it acts as an aspiration to ensure that we do meet 2C, then that will be good," he added.
Experts warn that to stabilise the climate at any temperature will require net zero emissions at some stage, which is included in the current deal as "reaching greenhouse gas emissions neutrality in the second half of the century".
It is likely to require action to plant forests, store carbon in soil or develop technology to capture carbon emissions from power plants and store them permanently underground, particularly to meet the more stringent 1.5C goal.
The level of ambition in the final deal being negotiated in the talks, which have almost reached the "end of the road", according to French foreign minister Laurent Fabius, who is chairing the talks, remains one of the key issues.
The issue of "loss and damage" - the recognition that some of the most vulnerable countries need support to cope with irreversible impacts such as inundation of their land from rising sea levels - is also hugely difficult, with developed countries refusing anything that could open them up to liability or compensation.
Finance for poor countries to deal with climate change and the different responsibilities of developed and developing countries to tackle climate change and pay for it, are also still the focus of political dispute.
It is thought there is movement on issues, however, that could lead to compromises, and that all countries still want to get a deal.
Nevertheless, Indian environment minister Prakash Javadekar accused developed countries of not showing flexibility towards poorer countries, and warned that unless rich nations showed the "spirit of accommodation", the success of the talks was not guaranteed.
Among other countries, a "high ambition coalition" including the EU, some of the world's poorest and most vulnerable countries and the US, Chile, Colombia, Mexico, Iceland and Norway has called for a robust climate deal, with Brazil the most recent country to join the group.
Marshall Islands foreign minister Tony de Brum, the driving force behind the coalition, warned there had been a "co-ordinated campaign to gut the text" of ambition by some countries, and pledged to fight for a strong agreement.
The coalition is calling for a robust five-yearly review system which would see countries review and increase their pledged emissions cuts, a move that is necessary as current climate action plans would only limit temperature rises to 2.7C.
Meanwhile US president Barack Obama has spoken on the phone to president Xi Jinping of China in the push to achieve an ambitious climate deal, the White House said.
The two leaders agreed the Paris conference was a crucial opportunity to galvanise global efforts to tackle climate change, and committed that their negotiating teams at the talks would continue to work closely together and with others to get an ambitious climate agreement, officials said.
Despite the overrun and the thorny issues that need to be tackled, eminent economist Lord Stern - who wrote the key review on the economics of climate change - said it was the best atmosphere between countries that he had seen in 10 years of attending UN climate talks.
He put the change down to the fact that countries were now "seeing a way forward" that could tackle climate change and help address other issues such as giving people access to electricity and boosting economic growth.