China’s agriculture ministry said on Wednesday it cannot rule out the possibility of new African swine fever outbreaks as concerns grow over the deadly disease’s spread in the world’s largest hog herd.

The ministry said in a statement on its website that it was not clear how widely the disease had spread, and there was much uncertainty on how the situation would develop.

China has reported four outbreaks of African swine fever in four provinces in less than a month and has culled more than 25,000 pigs, highlighting the challenge of containing the highly contagious disease.

The virus has been present and spreading in China’s neighboring countries for a long time, the ministry said. The risk of transmission remained large, it said.

China’s strain is similar to one that has hit Russia, Georgia and Estonia over the past decade, raising the possibility that it came across the border from Russia.

Beijing has not said how the virus reached China.

Beijing has asked local officials to stop transporting live hogs from high-risk areas and to tighten monitoring of hog transportation, the agriculture ministry said in its statement.

The rapid spread of the virus to places more than one thousand km (625 miles) apart may be caused by transport of pig products, rather than live pigs, said Juan Lubroth, chief veterinarian at the Food and Agriculture Organist (FAO).

“The movement of pig products can spread diseases quickly and, as in this case of African swine fever, it’s likely that the movement of such products, rather than live pigs, has caused the spread of the virus to other parts of China,” he said in a FAO statement on Tuesday.