(Adds witness quote and White House statement)
By Ian Simpson and Susan Heavey
WASHINGTON, Sept 16 (Reuters) - Several people were killed and others injured when at least one gunman opened fire at the U.S. Navy Yard in Washington D.C. on Monday, authorities said.
One Navy official said that four people had died and eight others were injured, but other officials suggested caution over those numbers saying the situation was in flux.
A Washington police spokesman said five people had been shot at the Naval Sea Systems Command headquarters, just 2.6 miles from the White House, including a District of Columbia police officer and one other law enforcement officer.
"There was three gunshots straight in a row," said Patricia Ward, who works at the Navy Yard, describing how she first heard the gunfire while having breakfast at the headquarters building.
A few seconds later, Ward said she heard four more gunshots. Security guards rushed in and got people out as fast as they could - "'Run, run, run,'they told people," Ward told reporters.
NBC News, citing a senior naval officer at the Navy Yard, said at least 12 people had been shot, with four killed and eight wounded. The report could not be independently confirmed.
A Navy official, who asked not to be identified, said several people were dead and several others were injured.
Another Navy official, also speaking on condition of anonymity, said more than one gunman may have been involved. The Washington Post reported that there were several gunmen.
The Navy said in a statement the shooting took place at the heavily guarded headquarters, where about 3,000 people work.
The shooting hit the military establishment less than three week after U.S. Army psychiatrist Major Nidal Hasan was sentenced to death for murdering 13 people in 2009 at Fort Hood, Texas, where he gunned down unarmed soldiers in what he later called retaliation for U.S. wars in Muslim countries.
A Washington police spokesman said the first reports of shots were received at 8:36 a.m. EDT (1236 GMT).
The White House urged citizens to follow instructions from autorities and first responders and said Predident Barack Obama had been briefed.
"The President has been briefed several times about the unfolding situation," a spokesman said. "The President directed his team to stay in touch with our federal partners, including the Navy and FBI, as well as the local officials."
Dozens of police and emergency vehicles surrounded the complex in southeast Washington, which is about a mile (1.6 km) south of the U.S. Capitol, local media reported.
Helicopters circled the headquarters with some touching down on the building's roof, according to a live feed by Washington's WJLA TV. The shooting briefly shut down flights at Reagan National Airport.
The Navy Yard dates to the 18th century and is the oldest Navy installation. It houses a museum, the residence of the chief of naval operations and is responsible for weapons development, among other functions. (Additional reporting by the Washington bureau; Editing by Grant McCool)