(Adds authorities do not expect spread, possible Czech source)

HAMBURG, June 25 (Reuters) - Six dead wild birds have tested positive in Germany for a lethal strain of bird flu, but authorities said on Monday they did not expect the disease to spread outside the southern region where it was discovered over the weekend.

On Sunday, three wild birds found dead in Nuremberg in the southern state of Bavaria tested positive for the deadly H5N1 strain of the disease.

The number of cases has since risen to six, with five swans and one goose infected, the Friedrich Loeffler Institute, a veterinary institution, said.

Authorities continued to investigate the outbreak, the first in Germany this year, which was discovered as part of a national testing programme for dead birds.

The government did not expect the outbreak to spread to other regions of the country, a spokeswoman for the Agriculture Ministry said.

Investigations were focusing on how the disease entered Germany.

It was possible it spread from the Czech Republic, where an outbreak was reported recently, the spokeswoman said.

"But this is only conjecture," she said.

Poultry farmers in the Nuremberg region have been ordered to confine all poultry to closed stalls. As of Saturday, a 21-day ban was imposed on moving poultry or poultry products in or out of the area, which is now a quarantine zone.

City officials also warned cat and dog owners not to allow their pets to roam freely in the quarantine zone.

Last year, some 13 European Union member states had confirmed cases of bird flu -- Germany, Austria, Denmark, Italy, Greece, Britain, the Czech Republic, Poland, Slovakia, Slovenia, Sweden, France and Hungary.

Bird flu has been spreading across southeast Asia, killing two people in Vietnam this month, the first deaths there since 2005.

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.

AlertNet news is provided by