More than 80 people have died in Ukraine in the last two weeks and there have been 450,000 cases of respiratory illness.
The World Health Organisation has sent an emergency team to monitor the epidemic and says it assumes most of the illness in the country has been caused by H1N1 swine flu.
The Ukrainian government has closed schools and universities for three weeks, and many public events have been banned.
President Viktor Yushchenko said experts fear the simultaneous existence of three types of flu viruses within the country "may lead to the emergence of an even more aggressive new virus".
The WHO team of nine experts dispatched samples from infected patients for examination at the World Influenza Centre at Mill Hill, in London.
"Many questions remain to be answered," the organisation said in a statement.
"The outbreak in Ukraine may be indicative of how the virus can behave in the northern hemisphere during the winter season, particularly in health care settings typically found in Eastern Europe."
The epidemic is concentrated in the west of the country, which has borders with four European Union states, but a growing number of cases are being reported in the capital Kiev.
The European Union is working closely with health experts in Ukraine, and member states are preparing to provide logistical and medical support if required...