A new round of talks on climate change sputtered to a close on Sunday, placing the onus on a UN summit in September to boost momentum for a global pact by the end of 2015.

Delegates reported faltering progress in the 12-day session, a waypoint towards a deal to keep climate-altering carbon emissions to safer levels.

"Political will needs to emerge at the New York summit," Martin Kaiser of Greenpeace told a press conference.

Taking place under the UN Framework Convention on Climate Change (UNFCCC), the negotiations seek to forge a historic deal in Paris in 2015 that will take effect from 2020.

Under the deal, 195 countries would make voluntary pledges on carbon gases so that warming does not breach a threshold of two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial times.

They would also channel financial aid to poorer countries to shore up defences against climate change and provide cleaner technology to help wean them off fossil fuels.

The haggling will move into higher gear from the second quarter of 2015, when the parties are supposed to have put their pledges on the table.

But before then, they have to agree common rules for vetting these efforts to ensure there is transparency and that pledge-makers are held to account.

"It's disappointing that negotiators didn't make more progress at this session on building greater consensus on the information that will be required in national contributions," said Alden Meyer with the US expert group the Union of Concerned Scientists.

"Many countries are already starting to work on their post-2020 contributions, and they need to have a sense of what information they will be expected to provide."

- Climate summit in New York -

The Bonn talks will be followed by UN Secretary General Ban Ki-moon's special summit on climate change in New York on September 23.

Heads of state and government will be scrutinised for how they intend to ramp up action. On June 2, US President Barack Obama set the ball rolling by unveiling plans for a cut in emissions of up to 30 percent from American power plants by 2030 compared to 2005 levels.

After the summit, a new UNFCCC round takes place from October 20-25, ahead of the forum's annual parlay, in Lima, Peru, from December 1-12.

European Union negotiator Elina Bardram said there had been a "good constructive atmosphere" in Bonn but Lima had to yield a template to make conclusion in Paris possible.

Ban's meeting -- the first top-level climate gathering since the 2009 Copenhagen summit -- is a "very, very important opportunity," she said.

The EU expected it to be a platform for "bold pledges" and a showcase for climate programmes that worked, she said.

On Bonn's plus side, co-chairs presiding over the negotiations agreed to issue by July 15 a new informal blueprint for a deal, said Climate Action Network, an alliance of green and development campaigners.

There were also advances in two technical arenas, on how to reduce carbon emissions from cities and using forestry and farming to help the fight against global warming rather than worsen it.

- 'Not enough on the table' -

"Though the progress here in Bonn by negotiators was heartening, there's not enough on the table. Heads of government (need) to get involved to make the tough choices negotiators can't," CAN said.

The Bonn talks revealed difficult and complex differences on the basics, delegates added.

Richer nations are focused especially on mitigation -- reducing emissions -- while poorer nations are more preoccupied for help with adapting to climate change, whose effects are already underway.

"Without finance on the table before Paris it's hard to see how we can avoid a stalemate, which puts a deal in danger," said Mohamed Adow of Christian Aid.

"Poor countries are asking developed nations to stick to their promises on finance before committing to cuts in emissions."

Another stumbling block is the negotiations' second track, which is about beefing up action on climate change before 2020.

Since the near-fiasco of Copenhagen Summit, trust in the climate process has been at a low ebb so promises of vigorous action in the short term are a key to cementing a a deal for the longer term.