Three people were shot outside a baseball stadium crowded with thousands of spectators in the US capital, causing the game to be abruptly halted as fans were ushered out.

Police initially tweeted four people had been shot, but Washington police second in command Ashan Benedict later said there were three wounded in last night's incident.

He said two of those hit were in one of two vehicles that was involved in a shootout. The third victim was a woman on the footpath outside the stadium where she had been attending the game.

"At no time during this incident were individuals inside the stadium attending the game in any type of danger. This was not an active shooter incident. Everything took place outside the stadium," Mr Benedict told reporters.

He said the woman was "expected to be okay."

AFP journalists who were at the game said some spectators had rushed for the exits after hearing a dozen or more gunshots outside the stadium, while others remained in their seats at the announcer's initial urging.

Several bursts of automatic gunfire could be clearly heard in the stadium in footage of the game posted on social media, while another video showed streams of people sprinting away from the stands.

The United States has a long and painful history of deadly gun violence, in the form of a steady daily toll of shootings as well as high-profile mass killings that have targeted schools, workplaces and shopping centres.

The lights of multiple police cars lit up the street and sirens could be heard as the crowd attending the game between the Washington Nationals and the San Diego Padres was asked to leave the stadium, in the Navy Yard neighbourhood of southern Washington, DC.

The department initially said two people had been shot, and then later added that further "victims associated with this incident walked into area hospitals for treatment of gunshot wounds."

The Washington Post quoted police as saying two of the wounded had non-life threatening injuries, with a man shot in the leg and a woman shot in the back.

The condition of the third person was not immediately known.

An intersection by the Third Base Gate of the stadium where the shooting occurred was sealed off by police.

The Washington Nationals' official account tweeted that a shooting had been reported there minutes after the incident took place, and encouraged fans to exit the ballpark.

Players had headed for the dugouts after the shots rang out and the game was suspended in the bottom of the 6th inning, with the Padres leading by eight runs to four.

The game is to be resumed today.

There have been just over 100 homicides in Washington this year, many of them involving firearms.

Police on Saturday offered a $60,000 reward after a six-year-old girl was shot dead the day before while riding a scooter in southeast Washington.