(Adds Dutch, Belgian comments paragraphs 16-19)

By Sybille de La Hamaide

PARIS, July 5 (Reuters) - Tests have confirmed that three swans found dead in eastern France were killed by the H5N1 bird flu virus, the French agriculture ministry said on Thursday, France's first cases of the disease in over a year.

The government raised its alert level to "high" as a result, meaning that birds and poultry in mainland France will either have to be locked up or protected by nets to avoid all contact with wild birds, a ministry official said.

Pigeon races and other events where birds are gathered will be forbidden.

"Michel Barnier, minister of agriculture and fishing, is putting in place the risk-prevention measures corresponding to the shift from the 'moderate' level to the 'high' level," the ministry said in a statement.

Germany said it was raising its assessment of the risk of bird flu following the French announcement and after officials on Wednesday discovered more birds that had died of the H5N1 virus, this time in the eastern state of Thuringia.

Spain's Ministry of Agriculture, Fisheries and Food on Thursday asked veterinary services to be more vigilant in the outlook for possible bird flu cases.

Spain already bans hunting with decoys, outdoor poultry raising and the isolation of breeding areas for wild aquatic birds in high-risk zones like bird migatory paths and wetlands.

In 98 regions in France classed as at risk -- some 15 percent of the country -- there will be special veterinary checks at poultry farms.

"EXTREMELY VIGILANT"

French Health Minister Roselyne Bachelot said on television that the country was not threatened by a flu pandemic at this stage but that the government would be remain on alert.

"We have to be extremely vigilant because the large flu epidemic that appeared after World War I, also known as Spanish flu, was of avian origin," she said.

In Britain, the government said there were no new measures in place yet although the situation in France was being monitored in close liaison with the European Commission.

"A preliminary outbreak assessment is under way and we are monitoring the risks. We would urge all bird keepers to maintain high levels of biosecurity and vigilance," a spokeswoman said.

The 1918-1919 flu killed at least 30 million people worldwide.

France, Europe's biggest poultry producer, increased its precautions against bird flu in June, saying the risk of the disease hitting the country had gone up after it was found in a number of wild birds in Germany and in the Czech Republic.

A French poultry breeders' group said in a statement that sanitary practices they put in place gave them sufficient tools to thwart the spread of the disease.

Dutch owners of commercial poultry have to lock up their animals as of Thursday, Agriculture Minister Gerda Verburg said in a statement.

Verburg advised non-commercial owners of poultry to lock up animals as well but vaccinated poultry were allowed outside.

Belgium's food safety agency AFSCA said in the event of a case in the country, poultry owners would have to confine their animals indoors within 24 hours.

It also asked the public to be on the alert for any suspect dead birds.

Last year, 13 EU states confirmed bird flu cases -- Germany, Austria, Denmark, Italy, Greece, Britain, the Czech Republic, Poland, Slovakia, Slovenia, Sweden, France and Hungary.

In France, the virus was found in more than 60 wild birds and at a farm with 11,000 turkeys. It had not been detected in the country since April 2006.

More than 30 countries have reported outbreaks in the past year, in most cases involving wild birds such as swans.

Globally, the H5N1 virus has killed nearly 200 people out of over 300 known cases, according to the World Health Organisation. None of the victims were from Europe.

(Additional reporting by Francois Murphy and Sonya Dowsett in Madrid, Emma Davis in Brussels)

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