The US Joint Base Andrews in Maryland has been placed under lockdown over an "active" gunman there, reports say.

Personnel were told to shelter in place, base officials said Thursday on Twitter following the lockdown just after 9 a.m. local time. 

The base, which is in Washington's Maryland suburbs, said on its Facebook page that the incident was ongoing at the Malcolm Grow Medical Facility. "First responders are on-scene now," it said.

The military base is located about 20 miles outside of downtown Washington, D.C. and hosts aircraft used by the US president.

Base officials said the facility had been scheduled to conduct an active shooter exercise, "however, reports of a real-world active shooter situation were reported" at the medical facility, according to Reuters.

"Due to the serious nature of this report, the base is reacting to ensure the safety of all personnel," it said.

Malcolm Grow is a full-service military hospital affiliated with the Uniformed Services University of Health Sciences and serves as a training facility for 48 medical residents and 31 health professionals.

The US military has been on high alert for possible attacks at its locations after incidents such as the July 2015 shooting that killed five service members at two military offices in Chattanooga, Tennessee, and the 2009 shooting at a US Army base in Fort Hood, Texas, in which a gunman killed 13 people.

Latest Update: Fresh accounts deny earlier reports of a gunman at the base, with US military officials saying all is clear there and that only a partial search is ongoing at the medical facility.

Officials say reports of an active shooter at a military post outside Washington stemmed from someone who made a distress call after seeing security forces doing a routine inspection. The confusion was heightened by a planned active shooter drill at Joint Base Andrews that had not yet begun and was planned for later in the morning. The base said in a Facebook post Thursday that there was no shooter and no threat to the base or workers there.