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 Thursday 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, 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.
The pact rests on voluntary carbon-cutting pledges from virtually every country in the world.
"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.
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.