Pledges under the UN from the world's major carbon polluters to tackle greenhouse-gas emissions are unlikely to be revised upward before 2020, a US climate negotiator said Monday.

"The idea that countries would change their current pledges that they listed in the [Cancun] agreements seems unlikely to me," Jonathan Pershing told journalists on the first day of UN climate talks in Durban, South Africa.

"I don't see the major economies shifting those actions," he added in a clear reference not only to the United States but emerging giants China, India and Brazil.

Nations accounting for more than 80 percent of global CO2 emissions registered voluntary reduction targets through the end of the decade at last year's high-level climate talks in Cancun, Mexico.

For wealthy nations, including the European Union and the United States, these pledges were expressed as an overall cut in emissions.

For developing countries, it generally took different forms, notably targets for improving energy efficiency.

Coming into the 12-day talks in Durban, some poorer countries -- many of them already suffering severe climate change impacts -- have called for more ambitious efforts for slashing CO2 emissions.

Current pledges fall far short of what science says is needed to prevent catastrophic warming.

But Pershing said some pathways to a climate-safe world were consistent with current commitment levels.

"Our thinking -- and that which we have heard explicitly from others -- is that there is no intention from other parties to modify the pre-2020 actions that they are taking," he said.

"It is in that context, of course, that we come to a post-2020 agreement."

The EU has proposed setting a 2015 target for hammering out a legally-binding comprehensive deal that would include all major emitters, to go into effect by 2020 at the latest.

Pershing acknowledged that greater efforts would be needed after 2020, but said the EU scheme put the cart before the horse.

"Some countries want to stipulate up front that such steps should be in the form of a legally binding agreement. Others -- including us -- have indicated that they want to know more about the content of such an agreement before they commit to a particular legal form," he said.

On the Green Climate Fund, which is to deliver 100 billion dollars a year to help poorer countries fight and cope with climate change, Pershing said US objections which had blocked progress could be resolved in Durban.

"We want to see the Green Fund move forward as part of a Durban package," he said.