Six people, including the suspected gunman, died Thursday following a "serious firearms incident" in the southwestern English city of Plymouth, police said, with reports one victim was a child aged under 10.

Devon and Cornwall Police declared a "critical incident" in the Keyham area of Plymouth early Thursday evening after the shooting, but later said it was not being considered terrorism related.

Officers responded alongside other emergency services, including air ambulance staff, to reports of gunshots in the residential neighbourhood near some of the city's docks.

"Two females and two males were deceased at the scene," Devon and Cornwall Police said in an update early Friday after hours of uncertainty around the incident.

Police added another male -- believed to be the gunman -- was also found dead nearby while another female treated at the scene died a short time later in hospital.

"All are believed to have died from gunshot wounds," the force said.

Shortly after police confirmed the death toll, local lawmaker Luke Pollard reported one of those killed "was a child under ten years old".

"More people are being treated for their injuries in hospital," the Labour MP tweeted.

"Just so unspeakably awful. My condolences and thoughts are with the families."

It was unclear how the victims and suspected perpetrator were connected, with many details of what unfolded in the city of around 262,000 residents in the sleepy southwest English county Devon remaining unclear.

"Police would stress this is not a terrorism related incident," Devon and Cornwall Police said, noting officers were not looking for anyone else in connection with it.

"Investigations are continuing," the force added, urging people with mobile phone footage of the aftermath of the events not to share it online.

Mass casualty shootings are rare in Britain, where most police officers are unarmed
Police Line | Representative Image AFP / Niklas HALLE'N

Interior Minister Priti Patel called the incident "shocking" and said her thoughts were "with those affected" but did not reveal further details of what had happened.

"I have spoken to the Chief Constable and offered my full support," she tweeted.

"I urge everyone to remain calm, follow police advice and allow our emergency services to get on with their jobs."

British media said residents had reported hearing loud bangs and gunshots before police swarmed the area.

Witness Sharron, who did not want to give her full name, told the BBC she had heard shouting followed by several gunshots.

"This was when the shooter kicked in the door of a house and randomly started shooting," she said.

"He ran from the house shooting as he ran and proceeded to shoot at a few people".

Robert Pinkerton, a second witness, told the British broadcaster he "walked around the corner" and "bumped into a bloke with a shotgun" dressed all in black.

South Western Ambulance Service said it had responded with a significant number of resources, including Hazardous Area Response Teams, air and ground ambulances, multiple doctors and senior paramedics.

Keir Starmer, leader of the Labour Party, said "tragedy has hit Keyham".

"I pay tribute to our emergency services who ran towards events we'd all run from," he added.

Mass casualty shootings are rare in Britain, where most police officers are unarmed, while southwest England -- a popular domestic holiday destination -- is typically known as a tranquil part of the country.

Plymouth, the region's biggest city, is home to the biggest naval base in western Europe, including nuclear submarines and large warships.