Australia’s Trade Minister Simon Birmingham says he is "deeply troubled” by reports that China has formally banned imports of Australian coal, in the latest sign the dispute between the nations is worsening.

More than 50 ships carrying Australian coal have been stranded off China after ports were verbally told in October not to offload such shipments. That ban appears to have been formalized, with the National Development and Reform Commission on Saturday giving power plants approval to import coal without restrictions, except from Australia, the Global Times reported.

If true that would "indicate discriminatory trade practices,” Birmingham said. "The risk profile of trading with China has grown significantly during the course of this year,” he said in an interview Tuesday with Australian Broadcasting Corp. radio.