China has denounced a landmark U.N. deal that caps emissions from international flights, in a setback for an industry eager to placate the growing international movement to curb air travel’s impact on the environment.

In a paper posted ahead of the U.N. aviation agency assembly that kicked off on Tuesday, China – once a critical early supporter of the 2016 U.N. plan – joined Russia in arguing the proposal would unfairly penalize emerging and developing countries because it raises costs.

The International Civil Aviation Organization (ICAO), which holds its assembly every three years in Montreal, set out the major climate initiative at its last full gathering in 2016, but aviation leaders are under pressure to do more after overall carbon emissions hit record highs last year.

Commercial flying accounts for about 2.5% of global carbon emissions, and its share of emissions is expected to rise as air travel becomes accessible to more travelers.

Swedish activist Greta Thunberg, who inspired a ‘flight-shaming’ protest movement against aviation, is expected to join a protest march on Friday in Montreal.

Reuters reports that the ICAO plan, known as the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) and the first of its kind for a single industry, is a medium-term scheme to help airlines avoid adding to their net emissions from 2020.

China has one of the world’s fastest-growing aviation systems and its participation in CORSIA’s first phase from 2021 is seen as critical for the deal.

“Given the difference among countries in development stage, historical responsibility and coping capability, the ‘one-size-fits-all’ approach for CORSIA implementation orchestrated by developed countries is a de facto reversion to the law of the jungle,” said the paper, which hardens the country’s public stance.

China’s absence could discourage other countries from participating. The United States, which left the Paris emissions deal in 2017, has said it supports CORSIA with the understanding “that it is applied by other major aviation states,” a U.S. State Department official said.

China joined the United States and Europe in 2016 to signal its support for the agreement but publicly cooled on the deal last year, declining to commit to a pilot phase.

A source said at the time China was likely distancing itself as a negotiating tactic.

The Chinese delegation could not immediately be reached for comment during the assembly’s opening day on Tuesday. But a Chinese delegation representative told the assembly that the country supports some kind of global climate plan for aviation.

A spokeswoman for the Russian delegation declined to comment.

Privately, China is taking preparatory steps that would allow it to implement the climate deal, which requires most airlines to limit emissions or offset them by buying credits from environmental projects.

It has asked for its environmental projects to be eligible for purchase by airlines through the program, said a source who spoke on condition of anonymity to discuss the private matter.

While China is a major source of emissions now, its relatively recent growth means its cumulative share of emissions is well below that of the United States or Europe.

Industry says around $40 billion in climate financing will be generated between 2020 and 2035 through the deal.

ICAO cannot impose rules, but sets standards for its 193 member countries that are widely followed.