Escambia County Sheriff's Office said in a message on Twitter it could "confirm there is no longer an active shooter on NAS Pensacola. The shooter is confirmed dead."

Naval Air Station Pensacola was put on lockdown, with the US Navy on Twitter reporting "one additional fatality." In a later tweet, it said "a second victim has been confirmed deceased."

The Pensacola News Journal said two people suffered critical injuries and a third non-critical wounds.

"There's probably been 100 or so various law enforcement vehicles" rushing through opposing traffic towards the base, Jeff Bergosh, a county commissioner who is a contractor at the facility, told the newspaper.

White House press secretary Judd Deere said President Donald Trump "has been briefed on the shooting at Pensacola Naval Air Station... and is monitoring the situation."

Broadcaster WEAR showed footage of an injured person being wheeled into a hospital, while the federal Bureau of Alcohol, Tobacco, Firearms and Explosives said its agents had responded to the incident at the base and "a search of buildings is being conducted."

- Training base -

The naval air station hosts 16,000 military personnel and more than 7,000 civilians, and is home to a flight demonstration squadron.

It is an early training center for naval pilots, and is known as the "cradle of Naval aviation."

Though relatively rare, military facilities have not been left untouched by the United States' epidemic of mass shootings.

On Wednesday, a US sailor fatally shot two people and wounded a third at the Pearl Harbor Naval Shipyard in Hawaii before taking his own life.

One witness from that attack told local media he was sitting at his computer when he heard shots fired and rushed to the window, where he saw three victims on the ground.

In July 2015, Mohammad Youssuf Abdulazeez carried out an attack at two military installations in the state of Tennessee that killed four Marines and a sailor, with the FBI concluding that the violence was inspired by a "foreign terrorist group."

Two years earlier, 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 being shot dead by officers.

Four years before that, Major Nidal Hasan, a US Army psychiatrist, killed 13 people and wounded more than 30 others at Fort Hood.

He was considered a "lone wolf" who supported terror network Al-Qaeda.