Russia expanded its ban on U.S. pork imports and threatened to ban U.S. pork altogether if producers continued to fall short of Russian safety and quality standards, Reuters reported Thursday.

Russian officials cited an excessive presence of the antibiotic oxytetracycline as the specific reason for the latest ban, though government representatives also pointed to wider disagreement over safety standards. 

“The [U.S. Food Safety and Inspection Service (FSIS)] has said it would not observe Russian food safety standards, without giving any explanation,” said Sergei Dankvert, the head of a Russian agricultural watchdog group.

“U.S. imports may halt for a certain period of time, as until now a mutually acceptable veterinary certificate has not been agreed,” National Meat Association head Sergei Yushin told Reuters last week.

“We are not closing Russia to U.S. imports, but the technical issue of the certificate exists,” added Yushin. 

According to Russian officials, the latest ban applies to two Smithfield Foods Inc plants, in Illinois and North Carolina, as well as Pork King Packing plant in Illinois and Hatfield Quality Meats in Pennsylvania.

Russia also recently suspended other Smithfield plants, in Iowa and Nebraska, after finding Salmonella and excessive oxytetracycline in pork from the two facilities.

According to Meatingplace, Tyson Foods’ in Nebraska and Sioux-Preme Packing Company in Iowa have also been blocked by Russian officials.

Russia is one of the top five markets for U.S. pork products.