A power outage swept the Washington area Tuesday, hitting the White House, the Capitol and the State Department and knocking out electricity for thousands around the US capital.

Outages stretched from downtown Washington into neighboring Maryland, knocking power out for more than 2,500 people, according to area power companies.

Most outages were brief, but computer systems were downtown offices and access to Metro trains were disrupted.

Washington power provider Pepco said the outage was caused by a dip in voltage as a result of an issue with the transmission line.

"There was never a loss of permanent supply of electricity to customers," Pepco said.

Electricity was back to normal by mid-afternoon and the company had dispatched teams to look into how it happened.

"Customers should be able to operate their own equipment at this time. There are no current supply problems. We have crews on site investigating the cause."

Outages were reported at more than 2,100 premises and households in Washington, according to Pepco, and more in Maryland suburbs southeast of the capital.

Major government buildings were not spared, including the White House, which lost power briefly.

Back-up generators kicked in promptly to restore lights and computers that were knocked out for several seconds, according to an AFP reporter.

White House spokesman Josh Earnest said there was no indication that the outage was as a result of a security breach.

"I do not currently see a nexus to terrorism," he told reporters.

The State Department went dark in the middle of a press briefing, which continued on for a time in the dark, a spokeswoman reading from her notes with the light from her cellphone.

"The State Department was among the buildings affected today by a power outage," spokesman Jeff Rathke said.

"The department has continued to carry out its essential functions throughout the outage."

Lights had returned to the building by mid-afternoon.

Power at Capitol Hill flickered on and off intermittently but was later restored, an official there said.

The Department of Homeland Security ruled out foul play, and said it was "closely monitoring the reports of power outages affecting parts of Washington, DC."

"At this time, there is no indication that this outage is the result of any malicious activity," DHS added.

Portraits in the dark

Elsewhere, several Smithsonian buildings were evacuated as they went dark, including the Air and Space museum and the American Art museum, which was closed. They later reopened.

The Portrait Gallery was also lights out, posting a photo of a darkened hall on Twitter.

"In case you wanted to see what a museum looks like when there's a power outage," the caption read.

Dan Kuntz, visiting from DuBois, Pennsylvania, was winding up his visit to the National Portrait Gallery when the lights went out.

"The power started flickering, and then boom," he told AFP as he enjoyed a soda on the steps of the National Air and Space Museum, which had also been affected.

Museum staff corralled most visitors into the open-air cafeteria space, said Kuntz, who himself made a bee line to the exit and proceeded to the National Museum of Natural History, which remained open.

Several metro stations lost power and were on emergency lighting, but trains continued to run in the nation's capital.

Metro police said on Twitter "units responding to assist at all stations affected by power outages. No elevator entrapments reported at any station."

Power had returned to most metro depots by afternoon.

The hashtag #dcpoweroutage was one of the top trends after the cut, with some users posting photos of darkened subway tunnels and the lights-out State Department briefing.

By later Tuesday afternoon, the regional power company, Southern Maryland Electric Cooperative, said only seven people were without power in the region.