All poultry bought at established markets, and properly cooked, is safe to eat, Yu Kangzhen, chief veterinarian at the Ministry of Agriculture, said on Monday.

"The source and transmission of H7N9 bird flu remain unclear, but the virus should be destroyed by high temperatures," he said.

The ministry said 20 variants of the H7N9 strain were detected from 738 samples taken at three live poultry markets in Shanghai on Thursday and Friday.

The variants are of low virulence and so far have not triggered an epidemic among poultry, Yu said, citing the latest test results.

But the possibility of the virus spreading to other kinds of animals and other regions also exists, he said.

The ministry urged veterinarians and workers involved in the breeding, transport, slaughter or sale of livestock and poultry to improve their protective measures against viruses.