The World Organisation for Animal Health (OIE) on Thursday said H7N9 bird flu posed a "rather 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 press release.
The 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 Paris-based 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 H5N1 bird flu.