Delegates to annual U.N. climate negotiations made only incremental progress Tuesday, even as researchers warned that if nations don’t bolster their plans to curb greenhouse gas emissions, much more costly reductions will be needed after 2020.

With just three days left in the U.N. Framework Convention on Climate Change meeting in Durban, South Africa, the major contributors to the world’s carbon output were divided over how to forge a more comprehensive approach to reducing emissions.

The fundamental sticking point at the talks is the same conflict that has dominated international negotiations for years: The existing global-warming treaty does not impose binding emissions cuts on some of the world’s top emitters, either because they were not originally bound or because they refused to ratify the agreement. Now, with the first commitment period of the 1997 Kyoto Protocol set to expire at the end of next year, delegates are wrangling over what sort of process should guide talks aimed at forging a new global warming treaty by 2020.

Jake Schmidt, international climate policy director for the Natural Resources Defense Council, an advocacy group, said in a phone interview that delegates are “stuck” on whether industrialized nations would adopt new climate targets under the Kyoto treaty starting in 2013 and what role the United States, China and India would play under a new climate framework.

The European Union has announced that it is willing to agree to a second round of emissions cuts only if the United States and major developing countries such as China and India sign on to a “road map” that aims to forge a binding agreement on reductions by the end of the decade.

Negotiators for China — which is the world’s top greenhouse gas emitter but is not obligated to make cuts under the Kyoto treaty — said publicly last weekend that they might be open to joining a legally binding treaty, but they have shown little willingness to make concessions in private sessions with other countries.

Connie Hedegaard, the European commissioner for climate action, tweeted in Durban: “Sometimes messages are more progressive at public press conferences than in meeting rooms.”

American officials have emphasized that they want a better sense of what a binding agreement would look like before they sign off on a process to create one. In a news briefing Tuesday, the U.S. special envoy for climate change, Todd Stern, told reporters that the United States “would be quite open” to discussing a process that might lead to a legal agreement, but he added: “We’re not prepared to say right now that it will be legally binding.”

The logjam has prompted leaders from poorer countries to question industrialized nations’ commitment to addressing climate change. Negotiators have made progress in sketching out how a new Green Fund for climate assistance for developing countries would operate, though they have yet to finalize details, such as how it would mobilize $100 billion in aid by 2020.

“Africa is not on the agenda for developed countries,” Africa Group spokesman Seyni Nafo said in an interview. “We’re facing an ambitions gap, we’re facing a financing gap, and we’re facing a legal gap if we don’t reach an agreement on a second commitment period under Kyoto.”

Meanwhile, two analyses this week show that even if nations meet their current climate pledges, drastic emissions cuts will be needed between 2020 and 2050 to prevent temperatures from rising more than 3.6 degrees Fahrenheit above pre-industrial levels.

Both Climate Interactive — a collaboration between the MIT Sloan School of Management and the consulting firm Ventana Systems — and Climate Action Tracker — a joint project of Climate Analytics, Ecofys and the Potsdam Institute for Climate Impact Research — called for nations to commit to more dramatic cuts in greenhouse gas emissions now.

Elizabeth Sawin, who helped conduct the Climate Interactive analysis, said that unless countries ramp up reductions today, “people need to understand it means turning off perfectly good multimillion-dollar investments before the end of their lifetimes.”

In a closed-door meeting with Stern on Tuesday, environmentalists asked whether the United States and other nations would be willing to commit to more ambitious cuts in light of the new findings. Stern replied it is unlikely that countries will revise their targets just one year after inscribing them under the U.N. climate-change convention.

William O’Keefe of the conservative Marshall Institute, speaking on WAMU (88.5 FM) and NPR’s “Diane Rehm Show” on Tuesday, said it is “the height of futility” to expect the world to transition to renewable energy more rapidly. O’Keefe, who also questioned the link between human activity and climate change, said the idea that political leaders can agree to cut global greenhouse gas emissions rapidly in the next few decades “flies in the face of reality.”