Police and local authorities rushed to the Walter Reed National Military Medical Center near Washington around 2.00 pm (1900 GMT) after someone activated a trial alert but failed to make clear that it was only a drill.

"The active shooter event... was the result of the improper use of a mass notification system by a tenant command aboard the installation," the Navy said in a statement.

"While preparing for an upcoming drill, the notification system was inadvertently enacted without containing the words 'EXERCISE' or 'DRILL.'"

The alert comes at a time of frequent mass shootings in America, and one week after a gunman killed three people at a Chicago hospital before police fatally shot him.

Distressed visitors and staff began messaging loved ones and tweeting about a possible mass shooter shortly after the alert went out.

The hospital itself was placed on lockdown and people told to "shelter in place."

"I am currently at Walter Reed Medical in Bethesda where we've been told there is an active shooter. I am currently safe in a conference room w/ approx 40 others," Congressman Dutch Ruppersberger wrote on Twitter.

The confusing series of events follows other communication blunders.

In 2016, US Joint Base Andrews outside Washington, the home base of the presidential plane Air Force One, was briefly locked down following a "miscommunication" about an active shooter training exercise.

In January this year a state employee in Hawaii triggered mass panic with a false alert of a ballistic missile headed for the Pacific islands.