FILE PHOTO: A cargo ship is loaded with coal during the opening ceremony of a new dock at the North Korean port of Rajin July 18, 2014. REUTERS/Yuri Maltsev/File Photo

SINGAPORE (Reuters) - China’s customs department has issued an official order telling trading companies to return their North Korean coal cargoes, said a trading source at Dandong Chengtai Trade Co., the biggest buyer of coal from the isolated country.

Following repeated missile tests that drew international criticism, China banned all imports of North Korean coal on Feb. 26, cutting off the country’s most important export product.

The source at Dandong Chengtai said the company had 600,000 tons of North Korean coal sitting at various ports, and a total of 2 million tons was stranded at various Chinese ports, waiting to be returned.

The source spoke on condition of anonymity due to the sensitivity of the subject.

Neither Dandong Chengtai (former Dandong Zhicheng Metallic Material Co., Ltd) nor Chinese authorities were available for official comment.

Shipping data on Thomson Reuters Eikon, a financial markets information and analytics platform, shows at least half a dozen general cargo vessels have recently taken coal out of China, mostly from the ports of Weihai and Peng Lai, and returned fully laden to North Korea.

Last month, Reuters reported that Malaysia briefly prevented a North Korean ship carrying coal from China from entering its port in Penang because of a suspected breach in sanctions. The ship was eventually allowed to unload its 6,300 metric tons of anthracite coal.