BERLIN (Reuters) - German authorities evacuated part of the United States consulate in Berlin on Thursday when an employee experienced breathing difficulties after handling a passport, but police said they could find no suspicious substances.

Police investigators in chemical protection suits and masks searched the building but pronounced it safe for staff to return to work after several hours.

An employee at the visa section had reported breathing difficulties and a metallic taste in her mouth after opening a passport handed to her by a male visitor, believed to be Albanian, police said.

The alarm came amid attacks on U.S. embassy and consulate buildings across the Middle East.

The Berlin consulate is in the southwest of the city, separate from the main embassy building in central Berlin.

Demonstrators attacked the U.S. embassies in Yemen and Egypt on Thursday in protest over a film they consider blasphemous to Islam, and American warships headed to Libya after the death of the U.S. ambassador there in related violence earlier in the week.

Islamist gunmen had staged a military-style assault on the U.S. consulate and a safe house refuge in Benghazi, eastern Libya. The U.S. ambassador Christopher Stevens and three other Americans died in the assault, carried out with guns, mortars and grenades.

(Reporting by Alexandra Hudson; Editing by Andrew Roche)