Pentagon on lockdown, firefighters sent to investigate incident that may have involved hazardous materials; CNN: False alarm

The Pentagon has been placed on lockdown, and firefighters wearing gas masks and protective suits have been sent to the headquarters of the United States Department of Defense (US) to investigate an incident that may have involved hazardous materials, US authorities said.

Citing sources familiar with the situation, the American television network CNN reported that the incident was attributed to a false alarm.

A Pentagon spokesman said earlier today that the Defense Department headquarters has "sophisticated systems in place to ensure the security of the building and its employees."

"These systems have detected an air quality issue that requires precautionary measures until we determine its significance," spokesman Sean Parnell said.

The department is implementing standard protective protocols, including a shelter-in-place order for the affected area. Emergency crews are on site and ready to assist employees in the building.

According to CNN, all access points to the building were closed, and employees were evacuated from several floors.

The US Department of Defense headquarters, which was damaged in the attacks of September 11, 2001, is one of the largest office buildings in the world.