Updated: 2.55pm

A lockdown on the US Capitol has been lifted, with police giving clearance for buildings on the Washington DC grounds to be re-opened. 

Visitors and staff were told not to leave or enter the site at around 9am (3pm Irish time) following reports that a person may have entered an office building with a gun. 

Police said they were looking for an individual but did not confirm the cause of the alert. One officer told AP they were responding to "sketchy reports" of a possibly armed woman. 

ABC News earlier quoted a Capitol Police spokeswoman as saying: "Out of abundance of caution, the US Capitol and Capitol Visitor Center are on lockdown while USCP investigates a report of a person of interest.

"Building occupants are advised to shelter in place and await further USCP instructions."

Congressional staff and reporters were told to close, lock and stay away from external doors and windows due to "police activity". 

Message from Capitol Hill police that the House press gallery just distributed regarding the lockdown: pic.twitter.com/OSkDAF6T7X

— Betsy Woodruff (@woodruffbets) July 8, 2016