Almost half of mainland China has been affected by a bird flu outbreak as the World Health Organisation (WHO) asked authorities to remain "vigilant" and employ emergency measures to prevent its spread. Chinese officials say the surge of the H7N9 virus, or avian influenza A, is still "controllable".

By February, bird flu had claimed 87 lives and its presence reported in 16 provinces and municipalities, according to the National Health and Family Planning Commission. The agency warned the infection could spread further if the situation is not controlled. The outbreak had killed 79 people out of the 192 reported cases in January. This is the worst surge of the virus in China since it first appeared in 2013.

The commission urged "utmost effort" from local authorities in curbing the spread and preventing more deaths. Some provinces in eastern, southern, and south-western China have totally shut down their poultry markets.

"It [the enforcement] is really strict this year. Vendors secretly sold live chickens when there was a similar ban before, but not this time," an anonymous vegetable vendor was quoted as saying by the South China Morning Post.

The virus is capable of causing severe diseases in poultry and necessitates close scrutiny, said the WHO. It said the virus is "highly pathogenic" among birds but said there is no confirmation on human-to-human infection. However, experts are worried that the virus could easily mutate and quickly spread among humans.

"This is the first time these changes have been detected. These are the only two cases in Guangdong province, China. So far, there have been no reports if similar changes have occurred elsewhere. It is a reminder that we have to keep looking closely," WHO spokesperson Christian Lindmeier told Reuters.