Japan offered $2 billion in aid on Wednesday to help developing nations reach species—preserving goals that are being debated at a U.N. conference, a move that could jolt the stalled talks forward.

With just three days left until Friday’s conclusion of the conference of the U.N. Convention on Biodiversity, delegates from 193 countries have made little progress toward reaching consensus on the meeting’s most contentious objectives.

The two—week conference aims to set targets for 2020 to slow or stop the alarming rate of extinction of plants and animals and damage to ecosystems. Scientists warn that unless action is taken to preserve species, extinctions will spike and the intricately interconnected natural world could collapse with devastating consequences, from plunging fish stocks to less access to clean water.

“We must stop this great extinction in our lifetime,” Japanese Prime Minister Naoto Kan said at the conference in Nagoya, southwest of Tokyo, in announcing the $2 billion aid offer over the next three years.

On Tuesday, a study published online in the journal Science showed that 1 in 5 of the world’s vertebrates, or animals with backbones, mammals, fish, birds, reptiles and amphibians, are threatened with extinction, although efforts to save endangered animals are helping.

But participants say that delegates in Nagoya have been unable to agree in two of the meeting’s prickliest areas, setting a target for protected marine areas and setting up a system for equitably sharing the profits from genetic resources, such as plants that Western drug companies have harvested to produce drugs.

Developing nations and indigenous groups have argued that they have seen little benefit from such resources, and delegates are seeking to create a legal framework for such “access and benefit—sharing,” in U.N. parlance, to rectify this.

Environmental ministers from the member nations were due to pick up the negotiations, some of which have been bogged down by concerns about how to pay for increases in protected areas.

Japan’s aid offer of $2 billion to help developing nations reach such goals, the biggest by far during the conference, could have “a catalytic effect,” said Russ Mittermeier, president of the environmental group Conservation International and a field biologist.

“The timing is excellent,” he said. Japan “really wants this meeting to be a success.”

Sue Lieberman, director of international policy at the Pew Trust, said the move could prompt other governments to step up with financial aid to keep the talks from collapsing, as the U.N. climate talks did in Copenhagen last year.

But Ms. Lieberman also pointed out that Tokyo has a dubious record when it comes to marine policies. It helped kill off many of the measures at the CITES, or Convention on International Trade in Endangered Species, meeting earlier this year that would have limited trade in tuna, sharks and other marine species. Japan has also come under harsh criticism by environmental groups for its whaling programme.

“Japan is not always the best friend of marine biodiversity,” she said.

Keiko Segawa, director of the public relations office at Japan’s Environment Ministry, rejected that description. As an island nation surrounded by the ocean, “we fully know the breadth of marine oceanology,” she said.

Delegates were still divided over how much of the world’s oceans to designate as protected by 2020, which can range from ocean sanctuaries to areas that have sustainable fishing.

Currently, less than one percent of the world’s marine areas are protected. Delegates are debating whether to raise that to 6 percent, a figure advocated by China, 10 percent or as high as 20 percent, Ms. Lieberman said.

American actor Harrison Ford, who has been on the board of Conservation International for more than 15 years, was also in Nagoya to encourage delegates to set ambitious goals.

“I just feel it’s an ethical responsibility to help do whatever I can to work for the benefit of nature,” Mr. Ford said. “I’ve got five kids and I want to see that there’s something left for them, enough of intact nature so that they can enjoy its beauty and benefits as my generation has.”