WASHINGTON (REUTERS, AFP) - The US Capitol, which houses Congress, was briefly locked down on Friday, with people directed to take shelter in their offices with locked doors and windows, as police searched for "someone" reportedly carrying a firearm.

Capitol police had ordered staff and visitors in the Capitol building and the adjacent visitor center to immediately take cover and to stay away from external doors and windows, a Senate statement said. A Capitol Police officer said an "event" was under way and authorities were looking for someone but offered no details.

The all-clear announcement was made shortly after 9:30 am (9:30pm Singapore time), about a half an hour after the lockdown began.

A Senate aide said police were looking for a woman who is possibly armed and in the basement of the Capitol building.

A Congressman was cited by the Hill, a US political website, as saying a Congressional staff member was seen with a firearm.

It was not immediately clear whether the police had found the person they were seeking.

The House Sergeant at Arms, responsible for security at the US Congress, confirmed the lockdown to AFP, but did not give a reason for the move.

The Associated Press cited a House of Representatives press gallery staff as saying the building was on lockdown but that the House met briefly and immediately recessed.

Voting scheduled to begin at 9.15am is postponed, the staff said.

Police later gave the all clear for the Capitol buildings to reopen.