Ministers from around the world need to make "crucial choices" together on a new climate deal, they were told as scores of countries lined up to back an ambitious agreement on global warming.
A new draft text of the agreement being negotiated at United Nations talks in Paris has been released, which still contains potential for ambitious targets on curbing rising global temperatures and cutting emissions over the coming decades, as well as weaker options.
But concerns were raised that there were not clear enough measures to achieve emissions cuts, with calls for a more robust "review and ratchet" mechanism that would see countries re-examining and raising the level of climate action if appropriate every five years.
French foreign minister Laurent Fabius, who is president of the talks, urged countries to scale up consultations on the key issues "to speedily come to compromises. We must altogether make these crucial choices".
The publication of the new draft comes after the United States aligned itself with the "high ambition coalition" of countries calling for a strong deal on tackling climate change at the negotiations by 195 countries.
The coalition, which also includes the European Union, Norway, some of the poorest countries in the world and nations including Mexico and Colombia, warned it would not accept a minimalist or bare bones agreement.
European climate action and energy commissioner Miguel Arias Canete said the current draft of the text was "not bold enough, and not ambitious enough".
In a speech to the conference, Mr Kerry said the situation and the moment demanded countries did not leave Paris without an ambitious and durable agreement.
"We need an agreement that is as ambitious as possible," he said adding that the US was "part of what we are calling the high ambition coalition".
"Unless the global community takes bold steps now to transition away from a high carbon economy, we are facing unthinkable harm to habitats, infrastructure, food production, water supplies and potentially to life itself," he warned.
Mr Kerry has joined calls for a review and ratchet system in the deal to increase emissions cuts by countries over time, saying: "It has to be an agreement that endures, it must get stronger over time.
"We didn't come to Paris to build a ceiling that contains all that we ever hope to do. We came to Paris to build a floor on which we can and must altogether continue to build."
Making sure ambition can be raised is key to achieving a 2C limit to global temperature rises - beyond which "dangerous" climate change is expected - or the more stringent 1.5C many countries have backed.
This is because the current pledges by countries for climate action they will take up to 2030 will only put the world on a path to almost 3C.
There is capacity within the draft deal for reviewing the climate action plans as early as 2018 or 2019, Michael Jacobs, senior adviser to the New Climate Economy project and formerly Gordon Brown's climate adviser, said.
He said the tougher 1.5C goal was still in two out of the three options for a temperature target, while strong language on the long term goal including net zero emissions, decarbonisation and climate neutrality all remained in the text.
The issue of "differentiation", the difference between the responsibilities and action of different countries, remains a thorny issue in the discussions.
Some countries, thought to include China and India, are trying to maintain the UN climate convention's original strict split between developed and developing countries laid down in 1992, while others such as Europe are seeking wording that reflects the range of countries' development and their ability to act.
Mr Jacobs said there would be a deal, though negotiations would be "tense and difficult" in the next two days, and there would not be a binary split between countries in the new agreement because it was too much of a red line for too many countries.