Migratory birds may have carried the H7N9 avian influenza virus into China, an expert with the Ministry of Agriculture (MOA) said Sunday.

Yu Kangzhen, the chief veterinarian with the MOA, said as the source of infection and modes of transmission remain unclear, the possibility of migratory birds carrying the H7N9 avian influenza virus into the country can not be ruled out.

The virus has mostly been found in live poultry trading markets, but not in farms raising poultry. The virus has not been found in pigs so far, he said.

The most infections have been reported in Shanghai, but the possibility of the virus spreading further also exists, he noted.

The MOA has dispatched work groups to local markets and farms to collect more samples. The ministry will publicize the test results in a timely and transparent manner.

Chinese health authorities said that as of 5 p.m. Sunday, 20 H7N9 cases have been reported in China, including six cases that have ended in fatalities.