* New global climate deal to be agreed by 2015
* Rules need to be less rigid, legal nature flexible-US envoy (Adds EU Commission comment, paragraphs 15-16)
By Nina Chestney and Valerie Volcovici
LONDON, Oct 22 (Reuters) - The United States called on Tuesday for a more flexible approach to a new United Nations' climate deal which balances the needs of all countries and has a better chance of success.
Two years ago, some 190 countries agreed to develop a pact to succeed the Kyoto Protocol which would force all nations to cut their greenhouse gas emissions. The deal is to be signed by 2015 and come into force in 2020.
Countries will meet again next month to work on the content and design of the new deal in Warsaw but progress this year has been slow.
Meanwhile, a scientific consensus that mankind is to blame for global warming has grown, putting pressure on governments - many of which have been focused on spurring weak economies rather than fighting climate change - to commit to ambitious emissions cuts.
In a speech at a conference at Chatham House in London, U.S. Special Envoy for Climate Change Todd Stern said for a new deal to be ambitious and fair it will require flexibility.
"A rigid approach is the enemy," he said.
Rather than negotiated targets and timetables, countries should be allowed to determine their own levels of commitment depending on their circumstances and means, Stern said.
This could be accompanied by a consultative period before the commitments are agreed to in which all countries and outside bodies can review them, and in which countries explain why their proposals are fair and adequate.
In 2009, the United States outlined a path for steep cuts in its emissions of 30 percent below 2005 levels by 2025 and 42 percent by 2030 but it has not yet reaffirmed this.
The deal will stand a better chance of working if the rules which govern it are less rigid, Stern said.
A very formal system based on rules for complying with targets and penalties for failing to meet them might sound good but would limit the ambition of many countries and their participation, he added.
The legal nature of the agreement also needs to be flexible.
Two years ago, countries did not specify precisely how a new deal would be legally binding, allowing for a "protocol, another legal instrument, or an agreed outcome with legal force."
Nations are in the early stages of discussing which elements of a new deal would be legally binding, but insisting that all aspects be internationally legally binding could be detrimental, Stern said.
Critics say that too much flexibility could soften the new agreement, making countries less inclined to pledge strict emissions cuts and/or stick by them.
"If we are all ambitious, why shouldn't we put our commitments in a legal document that binds us all? Whatever countries pledge must have the same legal weight," said European Commission spokesman Isaac Valero-Ladron.
The new agreement will replace the 1997 Kyoto Protocol, which bound about 35 industrialised nations to cut their emissions until the end of 2012. Last year, the protocol was extended to 2020, after which the new pact will take over.
Kyoto's effectiveness, however, was limited because it divided countries into rich and poor and developing nations such as China did not have binding emissions cut targets. The United States also never ratified the pact.
It is "unacceptable" to use the same categories in a new deal to determine who is expected to do what, Stern said.
China is now the world's biggest emitter and South Korea is now listed as one of the International Monetary Fund's 35 most advanced economies.
"Such a separation is inimical to ambition. It would also be viewed as deeply unfair by many countries, thus undermining the political cohesion we need to build an effective and durable climate system going forward," Stern said. (Additional reporting by Barbara Lewis in Brussels; editing by Jason Neely)
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