Two shooters opened fire at the US Navy Yard in Washington on Monday, killing four people and wounding several others including two police officers and one shooter.

Two shooters opened fire at the US Navy Yard in Washington on Monday, killing four people and wounding several others including two police officers and one shooter, officials reported.

Police and FBI agents descended on the area in force as helicopters buzzed overhead, amid reports the shooters were armed with assault rifles and were on the loose in the complex.

A Washington DC police officer and another law enforcement officer had been shot while the gunman had allegedly barricaded himself in a room in a headquarters building, the Washington Post and other media reported.

At one point a police helicopter hovering over the complex lowered a man down by rope into the compound.

Police blocked off intersections around the Navy Yard as military troops in uniform stood guard at street corners.

Crowds of onlookers stood on sidewalks watching the drama unfold and at a construction site near the Navy Yard.

"Four people were killed and several others were injured including the shooter in a shooting at the Washington Navy Yard," the Navy said in a statement.

"Emergency personnel are on scene and a 'shelter in place' order has been issued for Navy Yard personnel," it said.

Schools in the area were on lockdown as a precaution, authorities said, and flights out of the nearby Reagan National Airport were delayed.

The Navy said at least three shots were fired at 8:20 am (1320 GMT) in the headquarters building of the Naval Sea Systems Command.

It was unclear how armed men with assault rifles could have penetrated the heavy security that surrounds the Navy Yard, which is located on the Anacostia River, not far from the Capitol building that houses the US Congress.

One employee told the Washington Post he heard "blam, blam" inside one of the buildings, then someone pulled the fire alarm.

About 3,000 people work at the headquarters, which is responsible for building and buying US warships and combat systems, the navy said. The complex also has a residence which serves as the home of the chief of the US Navy.

The White House said President Barack Obama had "been briefed several times about the unfolding situation at the Washington Navy Yard by Assistant to the President for Homeland Security and Counterterrorism Lisa Monaco and Deputy Chief of Staff Alyssa Mastromonaco."

Defense Secretary Chuck Hagel and the US military's top officer, General Martin Dempsey, were also closely following the incident.

Flights from Reagan were suspended for 90 minutes, and the Federal Aviation Administration said it expected "residual delays as the airport returns to normal operations."