Pentagon lockdown has been lifted after a hazardous materials incident was declared a false alarm, according to US media reports.

The building was put on a lockdown after firefighters and rescue officials said there were investigating a hazardous materials incident.

CNN, citing unidentified sources, reported that emergency responders were wearing full gas masks and chemical protection suits.

"The Pentagon has sophisticated systems to ensure the safety of the building and its occupants. Those systems have detected an air quality issue necessitating precautionary measures until we determine its significance," Pentagon spokesperson Sean Parnell said in an email.

"The Department is executing standard protection protocols, including a shelter-in-place order for the affected area.

Response teams are in place and ready to support building occupants."

Floors two through five were locked down, as well as some corridors, CNN reported.

The five-sided Pentagon building, hit during the Sept. 11, 2001 Al Qaeda attacks, is one of the world's largest office buildings.