HAMBURG, June 26 (Reuters) - No new cases of the lethal strain of bird flu have been discovered in Germany since six wild birds tested positive for the disease over the weekend, Germany´s national animal disease institute said on Tuesday.

Six wild birds tested positive in Germany for the dangerous H5N1 strain of the disease. They were found in Nuremberg in south Germany and examined as part of a national testing programme.

"The number remains at six, we have no additional cases," a spokeswoman for Germany´s Friedrich Loeffler animal disease institute said.

Authorities continued to investigate the outbreak, the first in Germany this year, but the Agriculture Ministry believes the incident could be isolated.

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 bringing poultry or poultry products in or out of the area, which is now a 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.