British scientists are examining the strain of swine flu behind a deadly Ukrainian outbreak to see if the virus has mutated.
A total of 189 people have died and more than one million have been infected in the country.
Some doctors have likened the symptoms to those seen in many of the victims of the Spanish flu which caused millions of deaths world-wide after the World War One.
An unnamed doctor in western Ukraine told of the alarming effects of the virus.
He said: 'We have carried out post mortems on two victims and found their lungs are as black as charcoal.
'They look like they have been burned. It's terrifying.'
Neighbouring Poland has called on the EU to take action, fearing the mystery virus may spread westwards.
Prime Minister Donald Tusk has written to European Commission President Jose Manuel Barroso and the Swedish Prime Minister, Fredrik Reinfeldt, who holds the EU presidency.
The letter said: 'The character of this threat demands that rapid action be undertaken at the European Union level.'
Russia, Slovakia, Poland, Hungary and Romania have already launched health checks on Ukrainians entering their territory.
Slovakia has closed two of five border crossings.