US Secretary of State John Kerry on Friday urged signatories of an international ozone pact to back the phasing-out of hydrofluorocarbons (HFCs) -- toxic greenhouse gases thousands of times more potent than carbon dioxide.

"Climate change is happening - and it is happening quicker than most of us ever anticipated," Kerry said.

"Week after week, month after month, year after year, we continue to see new evidence, tangible evidence, of the danger climate change poses to our planet. Last year was the hottest in recorded history... but 2016 is on track to be even hotter."

He was speaking at a conference in Vienna uniting close to 200 signatories of the 1987 Montreal Protocol.

The Protocol, implemented by 196 nations and the European Union, commits all UN members to scrapping ozone-depleting chemicals known as chlorofluorocarbons, which were once widely used in aerosol sprays, solvents and refrigerants.

While the treaty has managed to slash these substances and help the ozone's recovery, it now faces a new challenge in the shape of HFCs.

As a result, nations have agreed to discuss an amendment to the Protocol to include the phasing-out of HFCs.

The Vienna conference was the last meeting before a final deal is due to be reached in October in Rwanda.

"HFCs... are exceptionally potent drivers of climate change - thousands of times more potent, for example, than CO2," said Kerry.

"Amending the Montreal Protocol to phase down HFCs is one of the single most important steps the world could possibly take at this moment to stave off the worst impacts of climate change."

Kerry said the move could help avoid a global temperature rise of 0.5 degrees Celsius (33 Fahrenheit) by the end of the century.

Under an international climate deal reached in Paris last year, 177 governments set a target of limiting global warming to "well below" 2.0 degrees Celsius compared with pre-industrial levels.