China is expected this month to formally submit its pledge for a global climate treaty that countries are seeking to finalize by December, the United Nations' top adviser on climate change said Thursday.

Chinese President Xi Jinping sent a letter to Ban Ki-moon last week about the issue, U.N. assistant secretary-general Janos Pasztor told The Associated Press. Pasztor did not give details.

China is the world's biggest emitter of heat-trapping carbon dioxide, and its pledge for the global treaty is highly anticipated. The United States made its own pledge in March, committing to a 26 percent to 28 percent cut in greenhouse gas emissions by 2025 from 2005 levels.

The climate treaty is supposed to be adopted at a Paris summit late this year. It would mark the first time all countries agree to do something to limit emissions of global warming gases, primarily carbon dioxide, from the burning of coal, oil and gas. France has pushed for a legally binding treaty, but that would have virtually no chance of being ratified by the Republican-controlled U.S. Congress.

Pasztor said 154 countries still have not made their pledges.

Xi last year said China will stop the growth in carbon dioxide emissions by 2030 and clean up its polluted air. He also has promised to double the percentage of renewable energy in China's energy sources by that year.

China is the world's biggest coal-consuming nation, using roughly half of all of the global coal production for power generation, heating and industry. But the government has said coal production and imports are dropping this year as it pushes to reduce carbon emissions.

So far, other countries that have made formal pledges for the climate summit include Canada, which last month said it plans to reduce its greenhouse gas emissions by 30 percent below 2005 levels by 2030. The EU has a target of 35 percent below 2005 levels by 2030, and Japan is proposing to cut its greenhouse gas emissions by up to 26 percent by 2030.

"What has been agreed by parties is that there will be no backsliding," Pasztor said. "Whatever countries put on the table, they should not be going backwards."

This week's U.N. negotiations in Bonn, Germany, are seen as a test of how close or far countries are from a climate deal. Pasztor didn't indicate any breakthroughs. "They're not going to negotiate away all the big issues at this session," he said.