(CNN) -- Tenants of apartments in Pacifica, California, were under evacuation Thursday after erosion threatened the cliff on which their building sits, a city official said.
Authorities have been watching the seaside building for years and were waiting for the erosion to reach a 12-foot safety zone behind it, said Doug Rider, a building official for Pacifica. The problem has reached that zone, he said, so officials notified the building's management and tenants began evacuating.
The building is stable, so tenants are able to remove belongings from the apartments, Rider said. Police, fire and other local authorities are assisting residents, along with the city's Building Department, he said.
All residents of the 12-unit building must be out by 5 p.m. PT Thursday, he told CNN affiliate KGO-TV in San Francisco, California.
KGO reported a large chunk of land fell from the cliff into the Pacific Ocean about 5:30 a.m. PT Thursday.
Two nearby buildings also face questions about their structural reliability but are not being evacuated, the station reported.
Pacifica is about 15 miles southwest of San Francisco.
CNN's Sara Pratley contributed to this report.