* Gases' effect can far outstrip that of carbon dioxide
* Deal gives three groups of countries different deadlines
* Key emitter India sought more time for developing nations

Nearly 200 nations hammered out a legally binding deal to cut back on greenhouse gases used in refrigerators and air conditioners, a Rwandan minister announced to loud cheers on Saturday, in a major step against climate change.

The deal, which includes the world's two biggest economies, the United States and China, divides countries into three groups with different deadlines to reduce the use of factory-made hydrofluorocarbon (HFC) gases, which can be 10,000 times more powerful than carbon dioxide as greenhouse gases.

‘It's a monumental step forward,’ US Secretary of State John Kerry said as he left the talks in the Rwandan capital of Kigali late on Friday.

As Rwanda's Minister for Natural Resources, Vincent Biruta, began spelling out the terms of the deal shortly after sunrise on Saturday, applause from negotiators who had been up all night drowned out his words.

Under the pact, developed nations, including much of Europe and the United States, commit to reducing their use of the gases incrementally, starting with a 10 percent cut by 2019 and reaching 85 percent by 2036.

Many wealthier nations have already begun to reduce their use of HFCs.

Two groups of developing countries will freeze their use of the gases by either 2024 or 2028, and then gradually reduce their use. India, Iran, Iraq, Pakistan and the Gulf countries will meet the later deadline.

They needed more time because they have fast-expanding middle classes and hot climates, and because India feared damaging its growing industries.

‘Last year in Paris, we promised to keep the world safe from the worst effects of climate change. Today, we are following through on that promise,’ said UN environment chief Erik Solheim in a statement.

GAINING MOMENTUM

The deal binding 197 nations crowns a wave of measures to help fight climate change this month. Last week, the 2015 Paris Agreement to curb climate-warming emissions passed its required threshold to enter into force after India, Canada and the European Parliament ratified it.

But unlike the Paris agreement, the Kigali deal is legally binding, has very specific timetables and has an agreement by rich countries to help poor countries adapt their technology.

The United Nations says phasing out HFCs will cost billions of dollars.

But a quick reduction of HFCs could be a major contribution to slowing climate change, avoiding perhaps 0.5 degrees Celsius (0.9 Fahrenheit) of a projected rise in average temperatures by 2100, scientists say.

Environmental groups had called for an ambitious agreement on cutting HFCs to limit the damage from the roughly 1.6 billion new air conditioning units expected to come on stream by 2050, reflecting increased demand from an expanding middle class in Asia, Latin America and Africa.

The HFC talks build on the 1987 Montreal Protocol, which succeeded in phasing out the use of chlorofluorocarbons (CFCs), widely used at that time in refrigeration and aerosols.

The aim was to stop the depletion of the ozone layer, which shields the planet from ultraviolet rays linked to skin cancer and other conditions.