SAN DIEGO - An "active shooter" was reported at a San Diego military hospital Tuesday, the facility in southern California said, warning people inside to take cover or fight back.
Cars enter Naval Base San Diego on May 8, 2015 in San Diego, California
"An active shooter has just been reported in building #26 at Naval Medical Center San Diego. All occupants are advised to run, hide or fight," the facility posted on its official Facebook page.
The nearby Naval Base San Diego also posted information online about a possible shooting.
"If in immediate danger, hide or seek shelter in secure space," the base wrote on Facebook.
"Barricade the door and dial 911. Silence phones and other devices."
According to a Navy statement, initial reports said a witness had reported hearing three shots in the basement of Building 26, which is a combination gym and barracks.
Navy spokesman Kevin Dixon told CNN the hospital was on lockdown, and responders were on scene along with military dogs.
San Diego, the southernmost city on the California coast, has a sprawling military infrastructure and is a major port for the US Navy.
The hospital is located in the city's Balboa Park, close to San Diego's famous zoo.
The United States has seen several deadly shootings at military installations in recent years.
In November 2009, US Army psychiatrist Major Nidal Hasan opened fire at his Texas military base, Fort Hood. He killed 13 people and wounded 42 more, before being overpowered by police.
In September 2013, Aaron Alexis killed 12 people and wounded eight others at the Washington Navy Yard, just two miles (three kilometers) from the US Capitol building, before he was shot dead by officers.