The clock is ticking for countries to lay the foundations of a 2015 deal to tackle dangerous climate change, ministers warned in Bonn on Friday.

A special UN summit in September, followed by a round of talks in Lima in December, must lay the first bricks of a highly complex accord due to be sealed in Paris in December 2015, they said.

China's top negotiator, Xie Zhenhua, pointed to traumatic memories of the 2009 Copenhagen Summit, the last time countries tried to forge a worldwide deal on curbing Earth-warming greenhouse gas emissions.

The much-touted event became a near-fiasco when heads of state were confronted with a sprawling, fiercely contested draft agreement at the last minute.

"We hope we do not see a recurrence of the Copenhagen scenario... (with) a final agreement that is accepted by some parties but not accepted by others," Xie said.

Peruvian Environment Minister Manuel Pulgar-Vidal, who will chair the December 1-12 meeting, urged colleagues gathered for an interim round of negotiations in Bonn since Wednesday to "commit to commit".

"This train is moving and we cannot wait until Paris to get onboard," he said.

The Paris agreement is meant to set the cap on years of haggling among the 195 parties of the UN Framework Convention on Climate Change (UNFCCC).

Taking effect from 2020, the pact must curb heat-trapping emissions from fossil fuels that are damaging Earth's fragile climate, amplifying risks from drought, flood, storms and rising seas.

Negotiators hope that the Lima deal, at the very least, will agree on rules for vetting and comparing national pledges that will form the core of the pact.

- Assessing the promises -

The pledges will be a disparate mix of promises to curb emissions, bolster climate defences, boost funds for vulnerable countries and transfer cleaner technology to the developing world.

There is a big technical challenge in ensuring that individual national efforts are verifiable and comparable, to avoid accusations that some countries are getting a free ride.

The pact's legal status -- whether it should be legally binding or not, and what the term actually means -- is also not settled. The United States, for one, has ruled out a format similar to the UNFCCC's Kyoto Protocol, which has a tough and legally enforceable compliance mechanism.

Xie said priority should be given to the content of the pledges, rather than the legal architecture. China would go along with the consensus, he said.

He stressed that a key to success in Lima, and then in Paris, lay in developed countries showing good faith in their pledges to act on climate change before 2020.

This includes a promise made in Copenhagen to channel up to $100 billion a year in aid by that time.

"Unfortunately we are seeing very little of the finance that was pledged," said Ugandan Environment Minister Ephraim Kamuntu, representing the world's bloc of least-developed countries.

The 12-day session in Bonn included two ministerial-level sessions meant to give a political boost to the troubled process.

Ministers from several dozen countries attended, but there were many absentees from big players, including the US and France, the 2015 host.

Many hopes ride on a summit in New York on September 23 called by UN Secretary General Ban Ki-moon.

Campaigners for 73 environmental and development causes, meanwhile, formally returned to the UNFCCC process after walking out at the annual conference in Warsaw in 2013.

The UN talks seek to limit warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels -- but scientists say current emission trends could hike temperatures to more than twice that level by century's end.

Veteran observer Alden Meyer of US group the Union of Concerned Scientists (UCS) said crunch time would come next year when national pledges are put on the table.

The White House, meanwhile, said that proposed US action against climate change would save thousands of lives by reducing asthma, heat-related illnesses and other health hazards.

Less than a week after President Barack Obama laid out his most ambitious moves yet to reduce carbon emissions, a White House report said: "we have a moral obligation to leave our children a planet that's not irrevocably polluted or damaged."

The US Environmental Protection Agency said Monday it would require states to take binding action to reduce carbon emissions from power plants by an average of 30 percent by 2030 from 2005 levels.