Chinese President Xi Jinping's trade policies are threatening to make his environmental goals more costly to meet.

The world's biggest gas importer on Friday included U.S. liquefied natural gas on a list of goods that could be hit with a 25 percent duty. While no date has been set to implement the tariff, the announcement comes just a few months ahead of winter, when Chinese demand for the U.S. heating fuel is likely to peak.

That Chinese policymakers would now take aim at U.S. LNG, which had been missing from previously targeted goods, signals that Xi may be willing to suffer some pain in order not to back down from President Donald Trump's escalating trade dispute. His government's clean-air push has made China the world's largest buyer of natural gas, and erecting barriers to U.S. supplies could force the country to pay a premium this winter.