China has detected an evolution in the H7N9 avian flu virus that is capable of causing severe disease in poultry and requires close monitoring, the World Health Organization (WHO) said on Tuesday.

Samples of the virus taken from two infected humans were injected into birds in a laboratory and became "highly pathogenic" for poultry, it said.

But that designation applies only to birds, not humans, WHO spokesman Christian Lindmeier said, and there is "no evidence that the changes in the virus affect the virus' ability to spread between humans."