At least three people were killed on Friday when a gunman opened fire at the Naval Air Station Pensacola, a US Navy base in Florida, officials said.
The gunman is also dead, the Navy tweeted.
An "unknown number of injured people [are] being transferred to local hospitals," the Navy added.
At least six injured people were expected at the trauma centre of the Ascension Sacred Heart Hospital Pensacola, spokesman Mike Burke said.
Seven people were being treated at Baptist Hospital, WEAR TV reported.
The circumstances of the incident remained unclear
Amber Southard, a sheriff's spokeswoman, said there was a gunman on the base in Florida's Panhandle. Within a few minutes, her office said in a statement that "the shooter is confirmed dead". The statement did not provide further information. It was followed up with a tweet by the Navy.
US President Donald Trump had been briefed and was monitoring the situation, a White House spokesman said.
On Wednesday, a sailor shot three civilians at the historic Pearl Harbor military base in Hawaii, killing two of them before taking his own life.
The Pensacola base, which is near Florida's border with Alabama, is a major training site for the Navy and home to its aerobatic flight demonstration squadron, the Blue Angels. The base employs about 16,000 military and 7,400 civilian personnel, according to the base's website.
A US sailor fatally shot three people and wounded a third at the Pearl Harbor Naval Shipyard in Hawaii on Wednesday before committing suicide, an official said.
The three people killed were Department of Defense civilian employees, while a third civilian was wounded but "in stable condition at (an) area hospital," the official told AFP.
Earlier, a spokesman for the US base would only say that security forces had responded to reports of a shooting at around 2:30pm local time and that the base was on lockdown for several hours.
One witness told local media he was sitting at his computer when he heard shots fired and saw three victims on the ground.
The witness, who did not want to be identified, said he then saw the gunman who was wearing what appeared to be a navy or sailor's uniform shoot himself in the head.
Hawaii News Now said several civilians were among the gunshot victims.
The shooting reportedly took place near the south entrance to the sprawling base.
