The United States ran into crossfire on Wednesday after it called for "flexibility" in climate talks yet acknowledged this may not guarantee meeting the UN's target on global warming.

Europe and Africa demanded that a two-degree-Celsius (3.6-degree-Fahrenheit) objective set at the 2009 Copenhagen summit be honoured while small island states, more vocal, accused Washington of backsliding.

The skirmishes came ahead of new talks in Bangkok starting at the end of the month for a global treaty to roll back greenhouse-gas emissions which stoke atmospheric warming, damaging Earth's climate system.

In a barely-noticed speech in New Hampshire on August 2, chief US negotiator Todd Stern said negotiations had to avoid a rigid formula that prompted nations to defend their own interests and avoid painful concessions.

Calling for "flexibility," he argued that a format that enabled progressively greater commitments would be easier to negotiate and ratchet up deeper cuts in the long run.

"This kind of flexible, evolving legal agreement cannot guarantee that we meet a two-degree goal," Stern acknowledged. "But insisting on a structure that WOULD guarantee such a goal will only lead to deadlock."

Stern's speech met with a hostile response from major parties in the climate parlay.

"World leaders pledged in Copenhagen to stay below the 2 C (3.6 F) temperature increase. What leaders promised must now be delivered," European Commission climate spokesman Isaac Valero Ladron said.

"Consolidated science continues to remind us of the dire consequences of going beyond such a temperature increase... Time is of the essence here."

Marlene Moses, chair of the Association of Small Island States (AOSIS), said Stern's speech "follows a well-established pattern of the United States lowering ambition at the climate talks.

"But it is particularly disturbing, coming as it does in the midst of one of the worst droughts in the country's history," Moses told AFP.

"If the US is prepared to abandon its own farmers, how are we supposed to believe it will do what is necessary to save small islands from sea-level rise and other devastating impacts?"

AOSIS, gathering low-lying nations in the Pacific, Indian Ocean and Caribbean, is campaigning for warming to be limited to just 1.5 C (2.7 F), a goal that could only be achieved with far tougher emissions caps than most states currently accept.

At present, Earth is on track for warming of 3-4 C (5.4-7.2 F) by century's end, scientists say.

Christian Aid's climate specialist, Mohamed Adow, accused President Barack Obama of retreating on a target that he himself had set in Copenhagen, where the figure was reached in chaotic scenes by a small number of world leaders.

"This backflip with a twist would win a gold medal at the hypocrisy Olympics," he said.

Addressing such criticism, the State Department quoted Stern as saying the United States "continues to support" the 2 C goal.

"We have not changed our policy," Stern said in this clarification. "My point in the speech was that insisting on an approach that would purport to guarantee such a goal -- essentially by dividing up carbon rights to the atmosphere -- will only lead to stalemate."

But the bloc of African countries in the talks said the clarification itself was a worry.

"It is concerning that the US would now question the global goal it pushed for, and has agreed to numerous times internationally," said Seyni Nafo of Mali, spokesman for the 54-nation African Group.

"It is more disappointing that in clarifying its position the Obama administration has said it 'supports' the goal but does not support an approach that guarantees achieving it."

The next round of talks under the UN Framework Convention on Climate Change (UNFCCC) run in Bangkok from August 30 to September 5.

They are the last scheduled negotiations under the 195-party UNFCCC ahead of its major year-end gathering, taking place in Doha, Qatar, from November 26-December 7.