The failure of wealthy nations to deliver on short-term climate commitments could hinder the rollout of a landmark treaty, a bloc of 134 developing countries, including India and China, warned at UN negotiations in Bonn.
The diplomatic spat has underscored the difficulty of reaching a consensus at the 196-nation talks.
"If we do not respect decisions that we have made, then how can we build trust among the parties?" said Chen Zhihua, China's senior negotiator, referring to long-standing pledges by rich nations to enhance financial support and "revisit" targets for curbing greenhouse gas emissions before 2020.
"And how can we lay a good foundation for the implementation of the Paris Agreement?" he added at a press conference Thursday, flanked by diplomats from India, Iran, Nicaragua and Ecuador.
The treaty, inked outside the French capital in 2015, calls on the world to cap global warming at "well below" two degrees Celsius (3.6 degrees Fahrenheit), and even 1.5 C if possible.
With one degree of warming so far, the planet has already seen an increase in drought, deadly heatwaves and superstorms engorged by rising seas.
The pact rests on voluntary carbon-cutting pledges from virtually every country in the world.
But those pledges are not enough to keep Earth in the safe zone, and would still see global temperatures rise a devastating 3 C (5.6 F) by century's end.
Moreover, they don't kick in until 2020, and developing nations say that's too long to wait to ramp up action.
"The science is clear: if we don't get our act together before 2020, you can forget about the 2 C and 1.5 C targets," said Paul Oquist, Nicaragua's chief negotiator at the talks.
"There has been a failure to comply with existing commitments," he added.
Under the terms of the UN's core climate convention, the burden for action before 2020 falls mainly on wealthy countries historically responsible for the rapid rise of greenhouse gases.
China is the world's top carbon polluter, followed by the United States, the European Union, India and Russia.
- Trust deficit -
Developing countries sought to have a "pre-2020 agenda" formally added to the negotiating process, but the move was shelved at the start of the 12-day talks. Efforts to resolve the issue have so far been fruitless.
"It would be a bad thing if this hangs over into the second week and becomes a political issue for ministers," said Alden Meyer, director of strategy and policy for the Union of Concerned Scientists in Washington DC.
"It has been a pretty sterile debate that has degenerated into a finger-pointing exercise," he told AFP.
Some 20 heads of state, including French President Emmanuel Macron and German Chancellor Angela Merkel, are scheduled to appear at the UN climate forum next week.
The European Union, Australia and the United States -- which continues to participate in the talks despite President Donald Trump's decision to pull out of the Paris pact -- have balked at training a spotlight on the issue, but are looking for a middle ground.
"There is no disagreement about the pre-2020 urgency," Elina Bardram, head of the EU's delegation for COP23, told AFP.
"But we must find solutions that ... do not compromise progress on the agreed negotiations programme" for the Paris Agreement.
For Teresa Ribera, director of the Institute for Sustainable Development and International Relations in Paris, the stand-off also reflects the negotiating process.
"It is in part tactical positioning to deflect mounting pressure" on some emerging economies -- China and India, in particular -- to deepen their own carbon-cutting pledges, she said.
Both countries are projected to easily meet their Paris targets.
But the poor nation-rich nation split that bedevilled these talks for many years has not entirely disappeared.
"This is creating a trust deficit," said Mohamed Adow, international climate lead for Christian Aid. "How can developing countries trust these very same countries that haven't taken seriously their previous commitments?"