All-clear for Greek man as experts scramble to contain bird flu in Nigeria

But just as Greek authorites said that a 29-year-old hunter hospitalised in the northern city of Salonika with avian flu-like symptoms had tested negative for the deadliest of the virus' strains, they also announced that cases of the broad H5 virus type had been found in birds in the north of the country.

Samples from two dead swans, one found in the sea near Salonika's waterfront, the other in a coastal area 90 kilometres (55 miles) from the city, have been sent to the EU Reference Laboratory for Avian Influenza in Weybridge, England to determine whether they were carrying the highly pathogenic H5N1 type.

The same laboratory was running tests on samples pouring in from both northern Greece and the southern island of Crete.

Since 2003, at least 90 people, mostly in China and Southeast Asia, have been killed by H5N1, which has been identified in northern Greece and the Aegean island of Skyros over the last week, as well as in Turkey, Iraq, Italy, Bulgaria, Croatia, Slovenia, Ukraine and Romania, where the country's 30th oubreak site of the H5 viral group was reported Tuesday.

The disease is highly infectious among birds, including poultry, and can be passed to humans. Doctors fear that if it mutates into a form transmissible among people it could trigger a flu pandemic and kill millions.

Bird hunting has been banned on Skyros and the Salonika region, while owners of poultry in both areas have been ordered to shut their flocks indoors.

Meanwhile in Nigeria, scientists were examining blood samples from farm workers as foreign experts arrived to help protect Africa from its possible first human cases of the H5N1 virus.

As experts from the US Centers for Disease Control (CDC) came to help study the outbreak, there were renewed fears the virus could spread into Nigeria's densely populated south after devastating northern chicken farms.

The health commissioner in the northern state of Kaduna, Mohammed Abubakar Bala, said tests were being carried out on workers from the site of Africa's first confirmed H5N1 outbreak among chickens.

CDC medical and veterinary experts from Atlanta and a laboratory team from the agency's centre in Kenya were due in Nigeria on Tuesday.

Nigeria last week announced Africa's first case of H5N1, which experts believe was spread by infected migratory birds.

Since then Nigerian veterinary scientists have confirmed its presence in flocks in three states and strongly suspect it has infected birds in five more, putting farms in a 280,000 square kilometre (108,000 square mile) patch of northern Nigeria at risk.

They also expressed fear that it should spread southwest, to the densely populated area around Africa's biggest city, Lagos, home to at least 16 million people, many of whom keep chickens in their back yards.

The surrounding countryside is dotted with big battery farms, including one owned by President Olusegun Obasanjo.

In the north, state and federal health officials have been moving between farms which have recorded suspicious poultry deaths and have killed, burned and buried tens of thousands of chickens, geese and ostriches.

Fears that bird flu may have already reached Niger, which lies north of Nigeria, were raised Tuesday by an expert from the UN's Food and Agriculture Organisation (FAO).

"There are suspicions for Niger and there is great agitation along the border between Nigeria and Niger," the head of the agency's contagious diseases service Juan Lubroth told a press conference in Rome.

He did not say whether analyses on dead birds were already being carried out in Niger, which borders the northern Nigerian states of Kaduna and Kano, where Africa's first cases of H5N1 were discovered last week.

Lubroth went on to warn that official restrictions on poultry movements in Kanu and Kaduna were not being properly implemented on the ground.

Residents of northern Nigeria have told AFP that dead chickens were still being sold in marketplaces and that chicken farmers were trying to move their production to neighbouring Benin.

The FAO also warned that when migratory birds return to Europe in a few weeks, there are likely to be fresh outbreaks.

Sweden's Board of Health and Welfare issued a similar alert.

"It is not unthinkable that we soon will have cases of H5N1 even closer to Sweden, and there is a risk that the virus will reach our country," the Board said in an article in the Dagens Nyheter daily.

Also Tuesday, the Italian Farmers' Confederation said plummeting sales of poultry and eggs would cost the industry "at least a billion euros", a quarter of turnover.