The remains of two swans found dead in Poland are undergoing tests to determine whether they had the H5N1 strain of bird flu.
Poland says it is a question of time before H5N1 reaches the country
If confirmed, it would mark the country's first case of the disease.
Initial tests have already shown the birds, found near the northern city of Torun, died from the H5 virus.
Polish authorities have been watching out for bird flu after outbreaks of H5N1 were reported across Europe in recent weeks.
The H5N1 strain can be caught by humans who handle infected birds, but is not yet known to have passed from one person to another.
Scientists have warned if it mutates, H5N1 could create a pandemic that could kill millions of people.
Samples from the swans were sent to an EU laboratory in Britain for testing on Sunday.
The deputy head of Polish veterinary services, Janusz Zwiazek, told AFP news agency it was "probably the H5N1 virus".
Poland's farm minister Krzysztof Jurgiel said results were expected by the middle of the week.
AFP said a "crisis unit" has been set up in Torun, about 200km (120 miles) north-west of the capital, Warsaw, adding that traffic restrictions had been introduced in the town as a precautionary measure.
On Saturday, Polish Prime Minister Kazimierz Marcinkiewicz said Poland was ready to tackle the situation.
"When the virus is discovered, which has to be expected, all the specialist services will be mobilised," AFP quoted him as saying.
Concerns were heightened after a dead swan, found to have been killed by H5N1, was discovered on the River Oder between Poland and Germany.
Germany confirmed its first cases of H5N1 last month
H5N1 has killed more than 90 people, mostly in East Asia, since late 2003.