BEIJING (AP) — China on Tuesday ordered importers of U.S. sorghum to pay deposits for possible higher tariffs in an anti-dumping investigation, adding to growing trade conflict with Washington.

A preliminary ruling by the Commerce Ministry said U.S. sorghum was being sold at improperly low prices, hurting Chinese farmers. It said importers must post bonds of 178.6 percent of the value of their goods to cover possible anti-dumping duties while the probe is completed.

Beijing launched the sorghum investigation on Feb. 4 in what some businesspeople saw as a warning shot as President Donald Trump hiked tariffs on Chinese-made washing machines and solar modules.

Since then, Trump has threatened to raise duties on up to $150 billion of Chinese goods in a dispute over technology policy. Beijing has responded with its own list of U.S. goods for possible retaliation.

The measures on sorghum, a grain used in animal feed and for making the fiery traditional Chinese liquor baijiu, target farm areas that voted for Trump in 2016. China is one of the biggest foreign markets for U.S. sorghum growers.

Investigators concluded dumping of U.S. sorghum "substantially damaged" Chinese competitors, the Commerce Ministry said. It said prices of U.S. sorghum fell 13 percent from 2013 to 2017, while shipments increased 14-fold.