Germany has confirmed that two swans found dead in the north of the country had the deadly H5N1 bird flu virus.
Infected birds have been found in five European countries this month
The birds were found on the island of Ruegen in the Baltic Sea, off Germany.
Austria has also confirmed its first cases of bird flu, in two dead swans. Sweden and Denmark have ordered poultry to be kept indoors as a precaution.
The United Nations food agency has urged governments in Western Europe not to panic, but warned the virus could spread further when birds migrate.
The H5N1 strain of the virus, which can kill humans through contact with infected animals, has recently been found in Italy, Greece and Bulgaria.
Further investigations are to be carried out on three swans found in southern Hungary to see if they died from H5N1. They have tested positive for an H5 subtype of the virus.
A spokeswoman for the UK's Royal Society for the Protection of Birds told the BBC News website it was probable the birds had shifted further westwards than normal in search for food following particularly cold weather in countries like Ukraine.
An exclusion zone has been imposed in the area where the swans were found in Germany. German health authorities are holding an emergency meeting to discuss the situation.
Germany's top medical research authority, the Robert Koch Institute, which carried out the tests, says there is no cause for panic in Germany.
Further tests on the dead birds are being carried out by a UK laboratory.
The same laboratory will test the swans found in Bacs-Kiskun in southern Hungary to determine which strain of the H5 bird flu virus they have.
The Hungarian authorities have restricted access to the two villages where the swans were found and put measures in place to prevent the spread of the disease.
Another eleven swans found in the north of Hungary near the border with Slovakia are being tested for the virus.
EU veterinary experts meeting in Brussels have backed plans to increase surveillance of wild birds and stricter bans on imports - particularly of untreated feathers from all non-EU countries.
The two-day meeting will examine additional measures that EU governments could take to stop the spread of the virus.
BIRD FLU IN EUROPE
August 2005: Avian virus reported in Siberia
October 2005: H5N1 kills thousands of birds in Turkey
Cases in Croatia and Romania
UK quarantines bird
December 2005: Ukraine cases
January 2006: Four people die in Turkey
Virus found in Cyprus
February 2006: H5N1 confirmed in Italy, Greece, Bulgaria, Germany and Austria
The German government announced on Tuesday it was bringing forward to next week a ban on keeping poultry outdoors, originally scheduled to come into effect at the beginning of March.
Further restrictions are now likely to be introduced, a BBC correspondent says.
Restrictions on poultry have already been brought into effect in Mellach, near Graz in Austria, where two cases of bird flu were found on Tuesday.
The authorities have set up a protection zone within a 3km (two-mile) radius of Mellach and a 10km surveillance zone beyond that.
Slovakia has banned the sale of poultry in markets following the Austrian cases.
The BBC's Christian Fraser, in Rome, says sales of poultry in Italy have fallen dramatically since H5N1 was discovered in eight dead swans, despite government assurances that it had not spread to chicken farms.
The Italian farmers confederation said eight out of 10 consumers had stopped buying chicken.
The federation said since October the poultry sector has lost some 600m euros (£410m) and 30,000 workers have been temporarily laid off.
While urging calm, the UN Food and Agriculture Organisation (FAO) warned that the virus could spread further into Europe as migrating birds return after wintering in Africa.
"We need to be aware that there's a real risk for Europe when the birds migrate northwards this spring," Samuel Jutzi, director of the FAO's animal production and health division, told reporters in Rome.
The H5N1 strain of bird flu has killed at least 90 people since early 2003, mostly in South-East Asia.
The virus can infect humans in close contact with birds. There is still no evidence that it can be passed from human to human.
BIRD-FLU OUTBREAKS AROUND THE WORLD
Confirmed H5N1 strain only
Human cases: laboratory-confirmed since Dec 2003
UK case discovered in quarantine, so disease-free status unaffected