A major US military hospital in a Washington suburb was on lockdown Monday as police investigated unconfirmed reports of an active shooter at the vast complex.

The US Navy said the Walter Reed medical center was "currently in lockdown and all base personnel are sheltering in place as a result of unconfirmed reports of an active shooter."

Local and military police were searching the facility, located in Bethesda, Maryland, northwest of Washington, it said.

On Twitter, Montgomery County Police said its officers had yet to confirm an "initial report of a single shot being heard."

"Out of an abundance of caution, MCP has additional officers responding to area to assist federal partners. No incident confirmed," it added.

Staff took cover as a "code white" lockdown was declared at the sprawling facility where 7,000 people are employed and security is described as tight.

One member of staff described colleagues and patients hiding in a bathroom with the lights off, amid shouts in the hallways for people to shelter in place.

Last Thursday, another major US military facility in Washington, the Navy Yard, was put on lockdown after an active shooter alarm that turned out to be false.

The Navy Yard was the scene of a mass shooting in September 2013 in which a lone gunman killed 12 people and injured three others.

Walter Reed is regarded as the top US military hospital, with patients ranging from Iraq war veterans to President Barack Obama.