The World Organisation for Animal Health (OIE) on Thursday said H7N9 bird flu posed an "exceptional situation" as the outbreak among Chinese poultry claimed a 10th human victim.
"Based on the information currently available we are facing a rather exceptional situation," said OIE chief Bernard Vallat, explaining that the virus, while dangerous to humans, was hard to detect in the host, which is farm birds.
"We are dealing with an influenza virus of very low pathogenicity for poultry which has the potential to cause severe disease when it infects humans," Vallat said in a statement.
The Paris-based OIE said that, based on reports sent to it by the Chinese veterinary authorities, infected birds "do not show any visible signs of disease, making it very difficult to detect this virus in poultry."
The OIE is the world's monitor for the health of farm animals traded across borders.
Past food crises it has handled include the mad-cow scare and the H5N1 bird flu, a different strain of avian influenza that first surfaced in Hong Kong in 1997.
On Thursday, China said the death toll from H7N9 reached 10 out of 38 confirmed human cases, with another fatality in Shanghai.
The virus is believed to spread to humans from birds, triggering the mass culling of poultry in several Chinese cities.
The fear is that the virus will mutate, making it transmissible from human to human.
The OIE said it had been informed of eight outbreaks of H7N9 in pigeons and chickens at Chinese markets, "all located in Shanghai and neighbouring provinces."
The possible reservoir for the virus in nature is being probed by the China Animal Disease Control Centre and China's animal health service, including a world-standard laboratory, the Harbin Veterinary Research Institute.
"At the international level, the Food and Agriculture Organisation of the United Nations (FAO), the World Organisation for Animal Health (OIE) and the World Health Organisation (WHO) are working together to support China's efforts to manage this new and evolving situation," the OIE statement added.