Nabarro said a lower death rate in Turkey, if confirmed, would not lower the risk of a human pandemic.
"Just because the death rate in infected people seems to be lower, it doesn't mean that we should necessarily be less worried. Maintain your vigilance, the mutation of the bird flu virus to become the human pandemic virus could happen at any time.
"It simply is telling us that the virus may be changing the way that it interacts with humans," he said.
"It does not tell us that that the risk of a mutation that causes the pandemic is increasing or decreasing."