Talk of swallowing dead rats and "circumcisions turning into amputations" has spiced up the fragile global climate change talks dragging on in Peru.

New Zealand lawyer David Tong, a civil society observer, is one of 14 young Kiwi observers tracking the negotiations at the United Nations Climate Change Conference.

Mr Tong said talks have gone on so long, negotiators from some of the poorest countries have had to fly back as they could not afford to stay in Lima or pay fees to change flight dates.

The UN talks started on December 1 and were supposed to conclude yesterday. The goal was to agree on what should be included in pledges submitted by countries for a climate pact which is due to be adopted in Paris next year.

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Mr Tong said the rift between developed and developing countries over how to tackle climate change was deep.

One sticking point related to "loss and damage" or how poor countries should be compensated for climate change-induced disasters, as well as long-term issues such as rising sea levels.

Some small island states at risk of inundation from rising seas complained a draft text from Lima did not include a loss and damage mechanism agreed upon in last year's talks in Warsaw.

Mr Tong said another sticking point was the concept of "common but differentiated responsibility".

This concept recognised the common heritage of humankind but also noted historical differences in the impacts of rich and poor states on the environment. It also recognised disparities in the abilities of rich and poor states to tackle global environmental problems.

"There's a very hard-line approach coming out of the US and others, including New Zealand, that there will be no mention of the term 'common but differentiated responsibility'," Mr Tong said.

Possibly complicating the issue was that some poor countries and some with low carbon emissions in 1992, when climate change talks began, were now much wealthier or had far higher emissions.

Mr Tong said China and India were among that group but were keen to now avoid having the same obligations as wealthier countries, as first defined in the 1990s.

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He would like New Zealand to recognise that some poor countries couldn't simultaneously afford to feed their own people and mitigate climate change.

"I was talking to one of the negotiators from the Marshall Islands today. He [said] half of the people in his country don't eat every day."

Mr Tong said these countries needed finance to help reduce their carbon emissions.

He said some very poor states suspected New Zealand had simply cut its aid budget and redefined it as climate finance.

Mr Tong said New Zealand did provide aid to some vulnerable, low-lying Pacific island states. "On that we're doing okay, except what we are not doing at all is championing their cause in negotiations," he said.

He said New Zealand would improve its global standing with a bolder emissions reduction goal. "We're a year away from Paris and we're refusing point-blank to up our target."

Mr Tong said talks had now entered a phase of "negotiation by exhaustion" as delays wore down delegates from the poorest countries.

It was possible there would be a total failure to reach any agreement, he said.

But Mr Tong said more likely, a "minimal text" favourable to rich countries would emerge. He said such a text would have little content or meaning but would help countries "save face" by not completely failing to reach agreement.

The New Zealand Government delegate in Lima, Jo Tyndall, this weekend said all countries were being asked to accept compromises.

"There are dead rats that need to be swallowed," Ms Tyndall reportedly said.

Vivian Balakrishnan, Singapore Environment Minister, also weighed in with what the Ecosystem Marketplace said "may be the most disturbing metaphor" of the talks.

"If you are submitting for circumcision, be careful it doesn't become an amputation because the surgeon used too big a knife and took too much flesh."

Ecosystem Marketplace reported the comment was also a metaphor for compromise.