WASHINGTON (Reuters) - A fire broke out during routine maintenance at the U.S. State Department on Saturday, injuring four maintenance workers, officials said.

The fire began inside ductwork on the building's eighth floor at about 11 a.m. (1600 GMT) as workers were replacing insulation, State Department and fire officials said.

Workers doused the fire with hand-held extinguishers before fire department personnel arrived, District of Columbia Battalion Chief Edward Mills said.

Mills, who was one of the first responders to the scene, told Reuters TV the cause of the fire remained under investigation but did not immediately appear suspicious.

As firefighters tackled the fire, State Department personnel were evacuated to a nearby facility, where around-the-clock operations continued, spokeswoman Victoria Nuland said.

"The fire was quickly extinguished and the main building has been reopened for normal weekend operations," Nuland said in a statement.

The firefighters' association Facebook page said three of the injured were taken to Medstar Georgetown University Hospital. A hospital spokeswoman could not immediately provide details about the conditions of the injured.

Washington Fire and EMS spokesman Lon Walls said the injured were all maintenance workers, and that one suffered serious injuries, according to the Washington Post. Walls did not immediately return a call for comment.

(Reporting By Chris Francescani; Additional reporting by Alina Selyukh and Peter Cooney; Editing by Tim Gaynor and Bill Trott)