SAN FRANCISCO (Reuters) - Police and federal agents converged on San Francisco General Hospital on Thursday after receiving reports of possible gunshots or explosions heard coming from the building, authorities said.

Members of the San Francisco Police Department's tactical operations unit swarmed the medical center on the edge of the city's Mission District and began a sweep of the hospital seeking to pinpoint the origin of the noise, police spokeswoman Giselle Talkoff said.

The hospital was not evacuated, but authorities sealed off the building to prevent anyone from entering or leaving as the search progressed, she said, adding that no injuries were reported

The investigation of possible shots fired was first disclosed by police in a message posted by the department's official Twitter account advising members of the public to avoid the area.

Agents of the Federal Bureau of Investigation joined local police in the sweep of the hospital, according to Prentice Danner, a spokesman for the FBI in San Francisco.

(Reporting by Curtis Skinner in San Francisco, additional reporting by Alex Dobuzinskis in Los Angeles; Writing by Steve Gorman; Editing by G Crosse and Sandra Maler)