While delegates expressed optimism that a deal was in reach, there was still horse-trading to be done, especially on the timetable on phasing out production of the gases.

US Secretary of State John Kerry was to meet with delegations from India and China, both major producers of hydrofluorocarbons or HFCs, whose agreement is crucial in reaching a deal.

Asked whether he was optimistic that up to 200 nations would agree to end the use of HFCs Kerry said: “We’re here to work for one. We’ll see what happens.”

These gases were introduced in the 1990s to replace chemicals that had been found to erode the ozone layer, but turned out to be catastrophic for global warming.

Banning HFCs — also found in aerosols and foam insulation — could slow the global temperature increase by half a degree Celsius by the end of the century.

Rwanda’s President Paul Kagame is hosting the meeting and on Friday was to meet Kerry, one of around 40 ministerial level delegates in his east African country’s capital. 

Opening the conference this week, Kagame said that eradicating HFCs “will make our world safer and more prosperous”.

While observers were optimistic as the high-level talks began Thursday, there was a change in mood in the afternoon as negotiations heated up and informal meetings dragged on late into the night.

“Time is running out, the pressure is rising,” said Maxime Beaugrand of the Institute for Governance and Sustainable Development.

Negotiators are weighing various proposals for amending the 1987 Montreal Protocol, implemented to protect the ozone layer, to freeze HFC production and use at some point between now and 2031.

India backs the later date, while other hot countries where HFC-using air conditioners are in high demand want temporary exemptions.

Developing countries like these are concerned about the high costs of the transition away from HFCs.

“We have come with an open mind and with the view that we must reach a fair and just agreement but what would a fair and just agreement imply?” said Ajay Narayan Jha of  India’s environment and climate change ministry.

“There are issues of cost, there are issues of technology, there are issues of finances. We would like to emphasise that any agreement will have to be flexible from all sides concerned. It can’t be flexible from one side and not from the other.”

Last month, a group of developed countries and companies offered $80 million (72 million euros) to help developing countries make the switch away from HFCs.

-‘Go the extra mile’-Despite the potential sticking points, Erik Solheim, head of the UN Environment Programme, said he remained optimistic.

“We are very close but of course we need to go the extra mile. If we make this agreement it will be one the most important global meetings in this year.

“We believe it can be a flexible agreement which takes into account the different views of the different parties, but also an ambitious one.”

HFCs predecessors, chlorofluorocarbons (CFCs), were discontinued under the 1987 Montreal Protocol when scientists realised they were destroying the ozone layer.

This blanket of gas in the upper stratosphere protects Earth from the Sun’s dangerous ultraviolet rays.

But it emerged that HFCs, while safe for the now-healing ozone, are thousands of times worse for trapping heat than carbon dioxide, the main greenhouse gas.

According to the Berkeley National Laboratory, air conditioning is the cause of the largest growth in HFCs — and the world is likely to have another 700 million air conditioners by 2030. 

Last year’s Paris climate agreement aims to keep global warming below two degrees Celsius, compared with pre-industrial levels. 

But continued use of HFCs could prove a serious stumbling block to attaining the goal.

HFCs — though they are greenhouse gases like carbon dioxide, methane and nitrous oxide — are not dealt with under the Paris Agreement but under the Montreal Protocol.

Any amendment to the protocol will be legally binding.

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