China is stepping up restrictions on the production, sale and use of single-use plastic products, the state planner said on Sunday, as it seeks to tackle one of the country’s biggest environmental problems. Vast amounts of untreated plastic waste are buried in landfills or dumped in rivers. The United Nations has identified single-use plastics as one of the world’s biggest environmental challenges. The National Development and Reform Commission and the Ministry of Ecology and Environment said plastic bags will be banned in all of China’s major cities by the end of 2020 and banned in all cities and towns in 2022. Markets selling fresh produce will be exempt from the ban until 2025.