In early August, just as protesters from across the country descended on North Dakota to rally against an oil pipeline near the Standing Rock Sioux Reservation, some of the world’s biggest banks signed off on a $2.5 billion loan to help complete the sprawling project.

Now, those banks — which include Citigroup and Wells Fargo of the United States, TD Bank of Canada and Mizuho of Japan — have come under fire for their role in bankrolling the pipeline. In an open letter on Monday, 26 environmental groups urged those banks to halt further loan payments to the project, which the Sioux say threatens their sacred lands and water supply.

In campaigning to reduce the world’s carbon emissions, environmentalists have increasingly focused on the financiers behind the fossil fuel industry — highlighting their role in financing coal, oil and gas projects. It is an expansion of traditional protest efforts, and it has met with some early success.

Environmental groups have also criticized the Dakota Access pipeline as outdated infrastructure with no place in a world racing to stave off the worst effects of climate change. The 1,172-mile pipeline is expected to carry nearly half a million barrels of crude oil daily out of the Bakken fields of North Dakota, according to the company building the pipeline, Energy Transfer Partners.