German authorities warn of swine flu mutation risk

* Expert sees autumn risk in northern hemisphere

* Merkel says Germany is prepared, coordinating with others

BERLIN (Reuters) - Germany's federal agency for infectious diseases said on Tuesday there were signs the H1N1 swine flu virus had started to mutate and warned it could spread in the coming months in a more aggressive form.

Experts were concerned about how the flu was developing in Australia and South America, said Joerg Hacker, head of the Robert Koch Institute for infectious diseases.

"It's possible the virus has mutated. In autumn the mutated form could spread to the northern hemisphere and back to Germany," Hacker told a news conference in Berlin.