Update 3.40pm: Law enforcement officials have given the base the all clear. No shooter has been found at the base.
All clear at Joint Base Andrews after lockdown, according to a law enforcement official.— CNN Breaking News (@cnnbrk) June 30, 2016
Earlier A US military base has been locked down amid reports of a gunman on the site.
Joint Base Andrews confirmed the lockdown on Twitter, and urged all personnel at the base in Washington's Maryland suburbs to take shelter.
Personnel have been advised to shelter and First Aiders are on the scene.
JBA is currently on lockdown due to a report of an active shooter. All personnel are directed to shelter in place. More info as it comes.— Joint Base Andrews (@Andrews_JBA) June 30, 2016
The base is located about 20 miles outside central Washington and is home to Air Force One and other aircraft used by President Barack Obama.
Rodney Smith, the patient advocate at the Andrews medical facility, said an active-shooter exercise was scheduled for Thursday morning, and then he was told it was a "real-world" situation.
"First it was an active-shooter exercise. Then it came back 'real world'," he said.
Mr Smith said the situation is unfolding at the newer of two buildings at the Malcolm Grow Medical Facility on the base. He is in the older building.
Homeland Security Secretary Jeh Johnson described it as an "unfolding situation", but provided no further details.
The incident is ongoing at the Malcolm Grow Medical Facility. First responders are on-scene now. All personnel continue to shelter in place— Joint Base Andrews (@Andrews_JBA) June 30, 2016