HAMBURG, June 27 (Reuters) - Germany must find the source of the outbreak of the lethal H5N1 strain of avian flu in wild birds, Agriculture Minister Horst Seehofer said on Wednesday.

Germany identified three cases of the H5N1 strain in wild swans on Tuesday in Leipzig in the eastern state of Saxony.

At the weekend, six wild birds in the southern city of Nuremberg tested positive for H5N1.

They were the first bird flu cases in Germany this year.

"The renewed appearance of bird flu in wild birds in Germany has surprised us in terms of the time," Seehofer said in a statement.

Despite the latest outbreaks, Germany has not raised its bird flu alert level, said Thomas Mettenleiter, head of the Friedrich-Loeffler national animal disease institute.

"We had always believed that the disease had not simply disappeared from us and this is reflected in the current risk assessment," he said.

German authorities said on Tuesday they were comparing the latest finds with viruses in outbreaks in Hungary and the Czech Republic to seek clues as to how the H5N1 strain entered the country.

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.