A top US Republican congressman was among several people shot when a man armed with a long gun attacked a Republican politician and staffer baseball practice session in the early hours of Wednesday morning in a Washington DC suburb.

Several Republican senators and representatives have described terrifying scenes of colleagues coming under heavy fire on the baseball field in Alexandria, Virginia, shortly after 7am, before the gunman was eventually brought down by local police officers.

Steve Scalise, a Louisiana congressman and the House Majority Whip, was among those shot during the attack, sustaining a bullet wound to the hip.

President Donald Trump confirmed the gunman died of his injuries several hours later. The FBI has said it is too early to say whether the ambush was an act of terrorism or an attempted political assassination.

The suspected gunman has been identified by multiple media outlets as James T. Hodgkinson, a 66-year-old American from the state of Illinois. Hodgkinson was a supporter of Democratic senator Bernie Sanders, according to his social media account, and had been photographed by a local newspaper in 2012 holding a protest placard saying 'Tax the rich'.

Reuters reported he had previously been charged with battery, though the charge was later dismissed.

US senator and former Republican presidential candidate Rand Paul was at the practice session, standing in the batting cage behind a tall fence, when the shooting began.

After the first shot, he said, "there was a rapid succession of shots, five or ten shots," he said.

"In the field I see Representative Scalise is shot but moving, and he's trying to drag himself through the dirt out into the outfield.

"There are two staffers... they were laying down. Then I'm seeing the shots hitting the dirt around them. They were kind of making a difficult decision - do we we lay here and stay low and hope he doesn't hit us... or does the shooter just advance and come closer and shoot you?"

Mr Paul said the gun sounded like "an AR15" but said he did not get a good look at either the gunman or the weapon.

Another senator described seeing a staffer attempting to flee the field after being shot.

"One staffer was shot out in the field, and he ran, with a wound, to the dugout," said Arizona senator Jeff Flake on CBS News.

"A lot of us dove into the dugout and tried to get as many [people] as we could. But at that point there was firing from behind us from the security detail, from the Capitol Police, and I started yelling back 'are you friendly?' Making sure that was our guy," he said.

"We didn't know if there were other shooters who had us surrounded... we didn't know whether to run or not.

Alabama congressman Mo Brooks said he heard 10 to 20 rounds from the gunman's weapon before the security detail returned fire. He said there were 20 to 25 members of team at the practice in Alexandria, when the gunfire erupted.

"I am absolutely certain it was a rifle... but I did not ever get close enough to it to identify the type of weapon being used," Mr Brooks said on CNN.

Mr Paul praised the actions of the Capitol Police in bringing the gunman down.

"It would have been a massacre without them," he said.

South Carolina congressman Jeff Duncan later said he believed he had an interaction with the assailant in the parking lot prior to the shooting taking place. Mr Duncan told reporters he was approached by a man resembling the gunman who asked him whether the players practicing were Democrats or Republicans.

According to his social media accounts, Hodgkinson was a supporter of former Democratic presidential candidate and independent senator Bernie Sanders, and had volunteered on his 2016 campaign.

Senator Sanders released a statement saying he was "sickened by this despicable act".

"Violence of any kind is unacceptable in our society and I condemn this action in the strongest possible terms," he said.

"Real change can only come about through nonviolent action, and anything else runs against our most deeply held American values."

Tim Slater, the FBI Special Agent in Charge of the investigation, told the media law enforcement was still trying to determine whether this was a targeted assassination attempt on the Republican congressmen or even an act of terrorism.

"It's too early in the investigation to say one way or the other," he said.

President Trump addressed the nation around five hours after the attack took place, praising the "heroic actions" of the two police officers who took down the assailant even after being shot themselves.

"Melania and I are grateful for their heroism and praying for the swift recovery of all victims," he said.

He called for unity in the wake of the "brutal, brutal assault".

"We may have our differences, but we do well at times like these to remember that everyone who serves in our nation's capital is here because, above all, they love our country," he said.

"We could all agree that we are blessed to be Americans, that our children deserve to grow up in a nation of safety and peace, and that we are strongest when we are unified, and when we work together for the common good."

Mr Scalise was struck in the hip and was taken to the hospital, where he is currently undergoing surgery, according to a statement from his office.

"Prior to entering surgery, the Whip was in good spirits and spoke to his wife by phone," the statement said.

"We ask that you keep the Whip and others harmed in this incident in your thoughts and prayers."

The politicians and their staffers were practicing ahead of the Congressional Baseball Game, an annual charity event where representatives from the two major parties play one another.

More to come

with Reuters