Europe and a group of developing economies traded barbs at the UN climate talks in Warsaw on Friday as frustration grew over slow progress towards a pact to tame global warming.

As the annual round of fractious negotiations lurched into extra time, European climate commissioner Connie Hedegaard accused a group of developing countries of opposing the "push" towards the deal, which must be inked in Paris in December 2015.

"There is still a group of like-minded (countries) who thinks differently, who tries to reinstall the firewall," she told journalists -- referring to existing arrangements that impose targets only on rich nations for cutting climate-altering greenhouse gas emissions.

"You'll understand that that is not acceptable to the European Union, but I also think to really many others, because they know that only if all of us do our utmost in the years after 2020, we will get what we really need."

Wealthy countries like the United States and Australia insist there must be no "firewall" in the new deal, which UN nations agreed in Durban in 2011 would include commitments for all.

Hedegaard did not mention the perceived culprits.

But her references pointed to a grouping calling itself Like-Minded Developing Countries, which includes China and India as well as Pakistan, Malaysia, the Philippines, Saudi Arabia and Venezuela.

They took affront at what they called Hedegaard's "brazen attack".

"It is incredible that she has chosen to accuse our group for blocking the process," said Venezuelan climate envoy Claudia Salerno, saying she spoke on behalf of the group.

"By going to the media and attacking negotiating partners, the EU chief is responsible for damaging seriously the atmosphere of confidence and trust in this process."

If the process is damaged as a result of Hedegaard's "incredible outburst", said Salerno, "she has to take the responsibility."

The issue of "equity" -- burden-sharing in curbing greenhouse-gas emissions -- has been a sticking point throughout the many years of climate negotiations.

Some developing nations, heavily reliant on atmosphere-polluting coal and oil for their economic growth, insist on having less onerous emissions curbs than wealthy nations.

They want the deal to impose "commitments" on the West, who they blame for creating today's problem through a long history of fossil fuel combustion, and to seek only "actions" from emerging economies.

The issue, as well as disagreements over funding to help poor countries deal with climate change effects, have caused the talks to run into extra time on Friday, and negotiators were preparing for a late night of haggling.

The process takes place under the banner of the UN Framework Convention on Climate Change (UNFCCC), which gathers 194 nation states plus the EU.

Developing countries have long negotiated as a bloc, but there have been recent signs of a split within it, between poor and highly vulnerable states and richer emerging economies.

The "like-minded" group, constituted in October 2012, says it gathers countries with 3.5 billion people, or roughly half of the world's population.