UN members on Thursday took their first steps in a marathon to negotiate a new global pact by 2015 that for the first time will place rich and poor under a common legal regime to tackle climate change.

Meeting in Bonn, the 195 parties to the UN Framework Convention on Climate Change (UNFCCC) began wrangling over how to work towards the target enshrined at their landmark conference in Durban, South Africa, last December.

Maite Nkoana-Mashabane of South Africa, who presided over the maiden session, urged countries as they embarked on the long road to set aside "old and unhelpful negotiating practices," a reference to the bickering that typically dogs climate talks.

"Time is limited and we need to take very seriously the desperate calls of some of our brethren, especially the small island states," she said, referring to low-lying nations threatened by rising seas.

The inaugural session of the ad-hoc working group, webcast over the Internet, took place in UNFCCC talks at senior level running in the former German capital to May 25.

If all goes well, a new accord will be wrapped up in 2015 and take effect in 2020, placing rich and poor under the same legal roof for tackling greenhouse-gas emissions that drive climate change.

At present, legal constraints under the UN's climate banner are divided among developed and developing countries -- a format dating back to the 1990s that critics say is badly out of date.

Rich countries bear most of the historical responsibility for global warming today.

But they say it is unfair to shoulder the burden for fixing the problem in the future.

Their places in the league table of emitters are being taken by emerging giants such as China, India and Brazil, which are massively burning coal, oil and gas as they battle to rise out of poverty.

Small-island states and African countries on Thursday sounded a loud alarm over the "ambition gap" -- the difference between pledges for cutting emissions and what is needed to avoid dangerous warming.

Scientist say current emissions are stoking possible warming of four degrees Celsius (7.2 degrees Fahrenheit), twice the 2 C (3.6 F) goal set by UNFCCC parties in 2011 as a safe maximum.

"The inadequate mitigation pledges... risk temperature increases that will have catastrophic impacts worldwide, and particularly for Africa," said Seyni Nafo, spokesman for the African group.

Speaking for small island states, Marlene Moses of Nauru warned that the "ambitions gap" was now so wide that by 2015, the negotiations could be dealing with how to relocate people from countries that had become inhabitable.

So far, the 2015 canvas is blank, except for the requirement that it meets the UNFCCC principle of "common but differentiated responsibilities." This term distinguishes between the demands that should be placed on poorer and richer economies respectively.

Who will curb their emissions and by how much, the pact's compliance regime and even its legal status are among the many issues that will have to be agreed in a hugely complex, multi-track arena.

Developing countries are demanding a show of goodwill from rich economies.

They want the European Union and its allies to renew and deepen their vows under the Kyoto Protocol, the world's only treaty that specifies cuts in greenhouse gases.

In contrast, the United States, which refuses to ratify Kyoto, is leading the charge for emerging giants to beef up their emissions pledges and open their vows to scrutiny.

In a sign of the tussles ahead, the UNFCCC said the first meeting of the board of the Green Climate Fund (GCF) -- an initiative designed to channel up to 100 billion dollars a year in aid to poorer countries -- had been postponed.

The May 31 meeting has been delayed "pending finalisation of the process for nominations" from countries seeking a place on the 24-seat panel, it said.

"The next window to meet will be the last week in June or first week in July, in Geneva," said Christiana Figueres, the UNFCCC's executive secretary.

"There is great enthusiasm from countries to be represented on the Board. While I would have liked to see the Board get down to work immediately, a short postponement to reach full agreement on its membership means it can launch smoothly and push ahead with the tasks before it."

The GCF was launched at the ministerial-level meeting in Durban. Germany, Mexico, Namibia, Poland, South Korea and Switzerland are bidding to host the fund.