Two policemen stand guard outside the Naval Medical Center in San Diego, California, the United States, Jan 26, 2016. [Photo/Xinhua]

SAN DIEGO - No evidence or signs of casualties were found after sweeps of a building at the Naval Medical Center in San Diego where an active gunshot was reported Tuesday morning, said a Navy commanding officer.

"A DoD (Department of Defense) employee reported the gun shot... the initial report was not through phone call, it was made in person," Capt. Curt Jones, commanding officer of Naval Base San Diego, said at a press conference in the afternoon.

"We are now clearing the building for couple of times... moving people out of the building, and we (have) not found anything which substantiates the earlier report," Jones said.

An active shooter was reported around 8 am local time on Tuesday morning in the southern California border city of San Diego. Local police officer confirmed a single witness reporting three gunshots fired in the basement of Building 26 of the center.

"An active shooter has just been been reported in building #26 at Naval Medical Center San Diego. All occupants are advised to run, hide or fight," the Center posted on its Facebook page.

"We don't take any of these calls as a joke, we take them very seriously," San Diego Police Department Chief Shelley Zimmerman told Xinhua.

"If somebody thinks they saw something that could potentially be very dangerous, be it an active shooter or something else, if they hear something, if they know something, we want everybody to say something," she said.

The medical center has more than 6,500 staff, including military, civilians, contractors and volunteers, according to its website. It spans 1.2 million square feet in the city's Balboa Park. On an average day there are thousands of patients visiting for medical or dental treatment.

San Diego is a border city with Mexico, and the Naval Medical Center in San Diego is the largest and most comprehensive military healthcare system in western United States. It has eight clinics for active duty personnel and nine primary care sites for their family members.