Russia has recorded the world's first cases of humans being infected with the H5N8 strain of bird flu.
The cases were confirmed by Anna Popova, head of consumer health watchdog Rospotrebnadzor, on Saturday.
Seven poultry workers in the south of Russia were infected in December but have since recovered and "the situation did not develop further", she said.
The cases were reported to the World Health Organisation "several days ago just as we became absolutely certain of our results", Ms Popova said.
She told Rossiya 24 state TV that there was no sign of transmission between humans so far.
Outbreaks of the same strain have been reported in Europe, China, the Middle East, and northern Africa in recent months, but only in poultry.
In December, the H5N8 strain of avian influenza was confirmed in rearing turkeys at premises near King's Lynn in west Norfolk, according to the Department for Environment, Food and Rural Affairs (DEFRA).
Other strains - H5N1, H7N9 and H9N2 - have been known to spread to humans.
According to the RIA news agency, Siberia's Vector Institute research centre will start developing human tests and a vaccine against the H5N8 strain.
The World Health Organization confirmed it had been notified about the cases, which it said were due to workers being "exposed to bird flocks".
It added that the workers were "asymptomatic and no onward human to human transmission was reported".
"We are in discussion with national authorities to gather more information and assess the public health impact of this event."
Most human bird flu infections have been linked to direct contact with infected poultry but properly cooked food is considered to be safe.
The disease is usually spread by migrating wild birds, so many countries keep their poultry production indoors or protected.