Russia accused a small U.S.-based charity on June 5 of "sabotaging" the construction of a huge gas pipeline to China and banned it as an "undesirable organization".

Jennifer Castner, director of the Altai Project, described the accusation as absurd but said it had been only a matter of time. The move followed clamp-downs on many foreign NGOs in Russia, including a similar ban last month on the local arm of the WWF environmental group.

The Russian prosecutor general's office said that while claiming to advocate nature conservation, the Altai Project was meddling in Russia's internal affairs and could damage its economic security.

"The key direction of the organization's work is sabotaging the construction of the Power of Siberia-2 gas pipeline," it said.

The planned pipeline is intended to deliver 50 billion cubic meters of natural gas per year from Russia to China via Mongolia.

Moscow put forward the idea many years ago, but it has gained urgency since Russia's invasion of Ukraine as it turns to China to replace Europe as its major gas customer. Beijing, however, has yet to sign off on the deal.

Castner's project supported local campaigns between 2009 and 2015, its website says, to reroute the pipeline away from Altai, a mountainous region in southern Siberia.

But Castner said it had not campaigned on the issue in recent years because the planned route no longer crosses the area.

The NGO's focus is on protecting the natural resources, wildlife and culture of Greater Altai region, including parts of Mongolia, Kazakhstan and China. Its projects have included conservation of snow leopards and rare birds of prey.

Castner said she is the only full-time staffer and the charity has modest funds—in the past two decades, it has disbursed more than $1 million to the region.

"We are simply providing support for communities to lead their own environmental activism and protect their own cultures," she said in a telephone interview.

She said the phone call from Reuters was the first she had heard about the ban and the accusations against her NGO.

"Of course it's absurd, but it's absurd in a terrible, damaging, awful way," Castner said. "All they're doing is hurting their own cultural and natural heritage."