BEIJING (Reuters) -China suspended on Tuesday the soybean import licenses of three U.S. firms and halted imports of U.S. lumber, stepping up retaliatory action after the United States imposed additional tariffs on Chinese goods. The three U.S companies affected by the license suspensions are farmer-owned cooperative CHS Inc, global grains exporter Louis Dreyfus Company Grains Merchandising LLC and export grain terminal operator EGT, China's customs department said in a statement. Customs said it detected ergot and seed coating agent in imported U.S. soybeans while the suspension of U.S. lumber imports was due to the detection of small worms, aspergillus and other pests.