By Agence France-Presse
China has culled more than 24,000 pigs in four provinces to stop the disease from proliferating, the FAO said in a statement. The first outbreak was reported in early August.
The FAO said the cases have been detected in areas more than 1,000 kilometres (600 miles) apart, meaning it could cross national borders.
"The deadly pig virus may spread to other Asian countries anytime," the FAO said.
The "diverse geographical spread of the outbreaks in China have raised fears that the disease will move across borders to neighbouring countries of Southeast Asia or the Korean Peninsula where trade and consumption of pork products is also high," it added.
China reported its first case of the disease in northeast Liaoning province earlier this month.
Last week, the eastern city of Lianyungang announced it had culled 14,500 pigs in an attempt to check the disease's spread.
"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," explained Juan Lubroth, FAO's chief veterinarian.
African swine fever is not harmful to humans but causes haemorrhagic fever in domesticated pigs and wild boar that almost always ends in death within a few days.
There is no antidote or vaccine, and the only known method to prevent the disease from spreading is a mass cull of the infected livestock.
In a report to the World Organisation for Animal Health, Beijing said an emergency plan had been launched and control measures taken to halt the spread of the disease.
The FAO warned in May of the risk of the spread of African swine fever from Russia.
Around half of the world's pigs are raised in China, and the Chinese are the biggest consumers of pork per capita, according to the FAO.